As I am about to start working on a Bus conversion, I'm finally ready to publish my research notes on choosing a bus for conversion. I'll have more to say later when I have more actual experience doing a conversion, but this material should be of some interest to anyone thinking of doing their own. I spent considerable time looking at buses and reading both school bus and professional bus driver forums before we made a purchase. Specific information on equipment will be largely applicable to the USA.
There are any number of potential considerations which could be involved in your decision. What matters or doesn't matter will vary depending on your lifestyle. Where are you planning to go, what are you planning to do, how are you planning to live, who are you planning to be around... All of these factors are going to affect your choice of vehicle. We wanted something that would look at much like an RV as possible, while still being as durable as a "Real" bus. Fuel mileage was a consideration, but not the most important one since we do not plan to move the vehicle often. Reliability and condition were paramount, as well as minimizing the amount of work we would have to do to the exterior. We settled on a 1999 Blue Bird Q-Bus because it had pretty much all of the equipment we had hoped for.
There are many types of potentially affordable buses. Short buses basically come in two types, either based on a cab and chassis van or truck or with a full body like a bread truck. Short school buses are usually extremely bare inside, while short shuttle buses are often carpeted and have fabric or carpeted walls. Midibuses (or mid-size buses) fall between those and a coach, and have about 12-28 seats. They tend to be based on light truck chassis, rather than a coach chassis, and have better fuel economy but are less durable. Transit buses tend run from about 30-45', with single rear axles, and are built much more stoutly. Sometimes they are even articulated, also known as "bendy buses", which are around 60' long; dual-articulated buses even exist, but they do not tend to be available cheaply. Coaches are designed for longer trips, and tend to have high-backed, reclining seats — and often, bathrooms. They are the tallest buses, and tend to be nicest-looking both inside and out. They often have extensive underfloor storage. School buses are generally short on amenities, but designed for durability and safety. They almost always have only one set of doors right at the front of the bus, so ingress and egress can be better-controlled by the driver. Activity buses are similar to school buses, but are painted differently and don't have flashing red lights on them. Prisoner transport buses are essentially activity buses with cages in them, and bars over the windows.
It is often possible to find inexpensive shuttle buses, usually based on a Ford E-350 or E-450 depending on their size. These buses may be designed to run on any type of fuel, depending on their environment. They tend to have decent interior, and often have shelves near the doors for the purpose of holding luggage. They are often painted white.
The cheapest full-size buses tend to be school buses, with shorter and/or smaller examples being cheapest of all. School buses are generally lighter and therefore slightly cheaper to run than other buses, and having only one door makes them good targets for conversion. Their yellow color picks them out immediately as a school bus, however. Some school buses do have additional doors to accommodate a wheelchair lift; chair lifts are rare on older full-size school buses, but they have become more common over the years. Most school buses have windows which drop partially for ventilation.
Transit buses can also be affordable, but are likely to be two to four times the price of a similarly-sized school bus. Transit buses are built heavier and often have at least front air suspension, for a better ride. The air suspension can also be adapted for use leveling the coach, and transit buses often incorporate a "kneeling" feature that lets the front end come down independent of the rear suspension (which may even still be spring-based.) Many if not most transit buses have wheelchair lifts, which are of interest both to the disabled and to people who want help lifting heavy stuff into their bus.
Transit buses typically have some type of opening window; often, the window slides horizontally, or sometimes there is a smaller window set into the glass which pops open. Transit buses are highly likely to have air conditioning. Transit buses which do not do dedicated shuttle or tour work often have a destination sign. Transit buses may have one, two, or even more doors (especially for longer buses.) Transit-style buses used as tour buses or shuttles may have only a mid-mounted door, only a front-mounted door, or multiple doors. Doors behind the front doors commonly only push out to open, and are intended only as exits.
Activity and Prison Transport Buses
Other buses which are of interest are "activity buses" and prison transport buses. Activity buses are basically white school buses without the stop sign and red flashing lights, and nicer seats. White is arguably the best color for your bus, and you could easily spend a thousand dollars just on the paint if you paint a bus yourself. They often have school bus-style windows, but they may also have transit-type windows. Most activity buses have a single door.
Prison transport buses are also usually white, and some of them have a toilet — often in stainless steel. Unfortunately, prisoners are not necessarily highly concerned about hitting the bowl, and such buses may have extensive corrosion and floor rot in the toilet area. Prison transport buses also tend to have steel cage partitions which have to be removed to make a nice living space, and they are usually riveted in. Removing them can be a significant undertaking. Prison buses usually have school bus-type windows, with horizontal metal bars over them to prevent inmates from escaping. If you are taking your bus to a potentially hostile environment, this is probably the best kind of bus for you, if you can find one in good condition. Older prison transport buses tend to be inexpensive, but even models over ten years old are often priced much higher than school or even activity buses.
Coaches, Double Deckers, etc.
Coaches designed for long-distance travel tend to be heavy, but also well-equipped. They commonly have an enclosed toilet. They usually have a destination sign, but do not necessarily. They also usually have a single front-mounted door. Most coaches have significant underfloor storage, so that they can conveniently carry large amounts of luggage, and a rear-mounted engine so as not to fatigue the operator with engine noise on long trips.
The only thing all buses have in common is large passenger compartments. Depending on their size, buses can be based on light, medium, or heavy-duty engines, with light, medium, or heavy-duty transmissions, and the engines can be mounted in the front, middle, or rear of the chassis. Rear-mounted engines face the opposite direction of mid- or front-mounted engines, and tend to feature the shortest drivelines. Front-mounted engines are usually accessed via a hatch next to the driver, with the result that they are very loud essentially at all times, but especially under load. Mid-mounted engines provide the best handling, but are most difficult to work on. Some rear-mounted engines are installed on rollers and actually come partially out of the bus for maintenance.
Buses themselves are either conventional (dog-nosed) or transit style (flat-faced). Conventional buses are universally front-engined, and the easiest to work on. However, this represents a substantial waste of space, and also makes it more difficult to see both the front of the bus, and any small obstacles that may be located there. Front-engined buses also have very long driveshafts, with multiple carrier bearings.
Most buses of today have at least a medium-duty engine. These engines are designed for extreme service lives of 500,000 active miles or more before a major overhaul. With proper maintenance (including fluid and filter changes) they can easily reach this sort of target. Short buses tend to have engines by Ford or GM, while larger buses usually have an engine made by Cummins, Caterpillar, International, or Detroit Diesel (also GM.) Each manufacturer of larger engines tends to make many models of engine, while Ford and GM generally make only one suitable engine per fuel type at once.
All heavy-duty engines have similar reliability, as they are engineered for similar lifetimes. They vary in their principle design, and in the cost of maintenance. In general, Cummins engines seem to be the cheapest to maintain, both in tool costs and part costs, and Caterpillar engines are most expensive. Until recently, almost all Cummins engines were inline six cylinder models, which are not only well-balanced but which also have fewer parts than eight cylinder engines. Cummins also seems to be the easiest engine manufacturer to work with, and is highly forthcoming with information.
To my mind the engine to have is either the 5.9 Cummins, the 8.3 Cummins, or the 10 liter Cummins, or as a runner-up the International DT466. The 5.9 Cummins (also known as the B series engine) is a medium-duty, "parent bore" engine; the 8.3 (C series) or the 10 liter (AKA "L10") are heavy duty models with "wet sleeve" designs. The DT466 is also a heavy-duty, wet sleeve design. Wet sleeve engines can be rebuilt in-frame if it ever becomes necessary, while "parent bore" (or "native bore") engines must be removed for reconditioning. Cummins engines from 94-98 may have a Bosch injection pump, which is preferable. 99+ models are electronically regulated, and the electronics make them slightly less reliable than earlier models which are purely mechanical in operation. 2003+ models of B series and 2004+ models of other engines have common-rail fuel injection, which is even more complex than the 99-02 models, but which offers more tuning potential. Besides fuel economy, the big advantage of the 5.9 liter Cummins engine is that it was also used in Dodge pickups for many years, and both parts and experienced service personnel are readily available. This same advantage also applies to the International T444, which was used in Ford pickups.
Most transit-style school buses have the 5.9 or 8.3 liter Cummins, or the DT466. Almost all front-engined models have the 5.9, while rear-engine buses may also have Caterpillar engines. Conventional school buses can have essentially any kind of engine in front, and are the most common place to find an International motor. Most transit buses have the 8.3 or L10 Cummins, or a Detroit Diesel V6 or V8, almost always in the rear.
Finding a short bus with a decent engine is a lot easier if it is a bread van sort of design than if it's a cab-and-chassis model with a Ford or Chevy snout and engine, but almost none of them have a Cummins. You just don't want a Chevy (the engines literally all have reliability issues) and you don't want a 6.0 liter Ford, either; you can make them reliable, but it's a lot of money. If you get a 7.3 Ford, you don't want it to be any older than 1995, when they went to the T444 engine — also known as the Ford Powerstroke. It's not actually a Ford anything, as they bought it from International. You also want it to have already had injector and HPOP service, because those things are expensive. However, with proper tuning they are capable of frankly amazing power output.
The pre-powerstroke 7.3 (AT185, etc.) is an inadequate design because it is just a bored out 6.9 and it has thin cylinder walls. Under load, the cylinder walls may flex by as much as .004"! This causes cavitation, where bubbles are created in the coolant. These bubbles create shockwaves when they collapse which can wear away at cylinder walls, leading to cylinder pinholing. I had one go bad on me in my 1992 F250, even though I was running the required coolant additive. Later models (1993/94) with factory turbo have a strengthened block which is less prone to this problem, but they only get into the bottom end of T444 output levels and should be avoided if at all possible.
Most buses in the USA which have automatic transmissions have one which was made by Allison. Allison transmissions are today grouped into series, referred to as 1000, 2000, and 3000. Most 1000-series and 2000-series you will find are four speed transmissions, though more recent models may be five- or even six-speed. All 3000 series transmissions have six gears, though it is common for one or even two of them to be "locked out". It requires substantial cooperation from both the builder and from Allison to have the lockout removed, although there are some companies which will sell you a replacement transmission control module for around a thousand dollars which enables the sixth gear. This is of interest because the sixth gear is a double overdrive, which not only lets you go a lot faster (which should be of little interest to most drivers) but also can provide a substantial fuel economy improvement while cruising on flat ground.
The most common automatic transmission on older buses is the Allison AT545, which is known to be somewhat unreliable. What's worse, it does not provide much engine braking down hills. Therefore, the transmission to have is either the Allison MT643, or anything in the Allison 3000 series. It is usually simple and relatively affordable to swap a MT in place of an AT. An AT545 is not necessarily a deal breaker, if you find a bus with either a Jacobs Engine Brake ("Jake Brake") or a transmission retarder, but you would be daft to buy a bus with an AT545 which didn't have one of those things. The B300 is a 3000 series transmission.
Short buses often have Allison transmissions, but as far as I can tell they are never 3000 series. They may well also have a Ford or GM corporate transmission. None of these are as good as a medium- or heavy-duty Allison.
Surplus short buses seem to be split fairly evenly between gasoline, diesel, and CNG. Diesel is the best option for mileage, and nearly as available as gasoline, though people in very cold climates may opt for gas to avoid gelling problems. CNG availability is poor, though it does exist. CNG buses can be based on gasoline or diesel designs.
Gasoline is in many ways the least attractive option for a bus for a broad variety of reasons, but primarily because the mileage is poor. However, gasoline is available at essentially every filling station everywhere. This makes it somewhat appealing, at least on short buses. One large problem with gasoline is that it has ethanol in it, and ethanol is hygroscopic. That means it pulls water out of the air. This water often causes damage to fuel components; while the problems are not as serious now as they were back in the age of carburetors, it is still an issue. Gasoline is also the most volatile of liquid transit fuels, and so it involves complex fuel systems including vapor recovery which are simply not present on diesels. Gasoline also spoils rapidly, even when fuel stabilizer is used.
While typical gasoline today has as much as 10% ethanol in it, E85 has around 50-80% ethanol depending on the season. E85 produces substantially less emissions than gasoline, but it also delivers about 15% less miles per gallon. E85 vehicles are also known as "flex-fuel" because they can run on any mixture of gasoline and ethanol up to about 85% ethanol. A fuel quality sensor measures the amount of ethanol and the engine is automatically tuned to run correctly on it. E85 also tends to be cheaper than gasoline due to ethanol subsidies, but this advantage is eliminated by the reduced mileage.
Diesel is generally the most attractive fuel for a bus. It offers both the best economy, and the least problems. Diesel fuel keeps for long periods, easily up to a year with no additives unless the fuel is contaminated with water. Diesel fuel stored for a long time with a lot of water getting into it is at risk of growing algae, which can clog fuel system components, but diesel algaecide (or "biocide") is available to solve this problem. Water in diesel fuel can also cause harm to your engine, but diesel fuel systems include water separators to address this issue. In very cold climates, diesel fuel will gel, and fuel heaters are necessary to get the engine going, which is probably the best reason that someone might select a gasoline-powered bus. Once the engine is started, however, fuel returned to the tank from the injection system keeps the fuel warm enough to be pumped and injected. This excess fuel is heated as it cools the injection pump, injectors, and related components.
Diesels have better fuel economy than gasoline engines because they always run lean, which is also the reason they produce oxides of nitrogen ("NOx".) To combat this tendency, catalysts similar to those used on gasoline vehicles was introduced, and later, urea injection was created. Diesel catalyst systems usually inject some diesel fuel into the catalyst in order to burn off soot deposits, harming fuel economy. Urea injection systems use "DEF", or diesel exhaust fluid, in order to react nitric oxides. They are common points of failure, either at the pump or injector. It is illegal to defeat these systems if installed at the factory, but RVs do not have emissions test so there is no need to retrofit them as has become necessary on commercial vehicles including school buses.
Biodiesel and Green Diesel
Biodiesel is a diesel fuel produced by "transesterification" of lipids, or fats. The process can use either plant or animal fats, but plant fats are more commonly used because they result in a fuel with a higher "gel point". Biodiesel has very high lubricity, but if improperly made it can also have high acid content which will damage fuel system components. It is commonly run as a blend, which is named for its percentage of biodiesel content; B20, for example, has 20% biodiesel, while B100 is pure biodiesel. Putting just 5% biodiesel into your fuel will increase fuel lubricity (and thus extend the life of fuel system components) more than any other additive, while presenting essentially zero risk. It also helps clean the cylinders. Biodiesel produces slightly less power than normal diesel fuel, but significantly lower emissions. It does however tend to have a higher gel point temperature than petroleum diesel, especially when made from animal fats.
Green Diesel, on the other hand, is diesel which has been hydrocracked. This is essentially the same as the process by which petroleum is refined into gasoline and diesel fuel (both of which are produced in that reaction.) While this is a more energy-intensive process than that which makes biodiesel, such fuel can be used in any proportion (including 100%) in any diesel vehicle. In spite of this, it still provides more power than ordinary diesel, while also lowering emissions. It also has no gel point problem, or at least, not more severe than petroleum-derived diesel fuel.
Compressed natural gas is the same stuff that you use in your stove or furnace, but instead of being supplied to the home at single digits of PSI and to appliances at fractional PSI, it is stored at thousands of PSI. It is the least readily available but cleanest fuel, to the point that it actually puts less contamination into the engine oil which can extend oil change intervals. You can supposedly buy it at UHaul facilities. CNG fuel systems involve high pressures and gaseous fuel, and they offer many opportunities for problems.
Almost all full-width (8') commercial buses of about 30' or larger have air brakes, which do a better job of stopping buses and are not going to leak hydraulic fluid — but if you're the type who likes to step on the brake pedal often, you should have hydraulics because that guy is going to wind up depleting his air supply and having the brakes lock up until the compressor can catch up. Air brakes also depend on the health of the air supply system, which notably includes an air compressor. The air system contains multiple reservoirs, usually five or six of them, and these must be occasionally purged of water even though these systems also include an air dryer. This sometimes requires crawling under the bus, but there may instead be service valves behind a panel on the exterior.
With that said, hydraulic brakes can also have many problems. The brake servo (or "booster") often fails, and they often leak fluid. Bleeding brakes even on ordinary automobiles can be a hassle, and on a longer vehicle like a 30' bus it can be a chore. It is probably preferable to have air brakes on any sizable bus.
Other than being more durable than typical RVs, one major appeal of surplus buses is their low cost. While an RV with a flimsy fiberglass exterior typically costs $50,000 and up for anything that doesn't leak, I've been seeing credible school buses recently taken out of service (meaning recently maintained) sell for around $3000, with decent tires on them no less. I've been seeing good-looking transit buses sell for around $10,000. You can essentially spend as much as you want on pretty much any kind of bus, however; it is not uncommon for a new transit bus to cost over $300,000, and the closer a bus is to new, the larger percentage of the new price you will expect to pay. Coaches can easily cost as much as an RV, or even more.
Parts costs also must be taken into account when choosing a bus. Unless you are getting a whopper of a deal, at these prices even only moderate repairs can easily be as expensive as purchasing the bus itself. A rebuilt 8.3 liter Cummins engine can cost around $10,000 installed alone. However, it seems you can get rebuilt MT643 transmissions for around $650 at this time, so not every major system is necessarily expensive. Missing or damaged glass may be difficult to source depending on who the manufacturer was, and whether they are still in business — not a major concern for newer buses, but still worth mentioning.
I haven't even got into maintenance yet, but there are some obvious considerations you should take into account. If you will be going off the beaten track at all, you will need a spare tire and some way to change it. If you don't feel up to making a tire change using big tire irons, you're going to need it mounted on a wheel. A commercial truck-sized tire+wheel combo is extremely heavy, and many if not most people will be incapable of making a change on their own. You also need a 3/4" drive torque wrench capable of at least 250 ft-lb, and maybe as much as 500 ft-lb or more in order to properly torque the lug nuts. You'll also need a stout jack to raise the vehicle for the change. Properly maintained bus tires rarely exhibit problems, but that still means that flats do happen sometimes. This alone may recommend a smaller bus.
Another reason you might want a smaller bus is the cost of scheduled maintenance. While short buses will use amounts of oil and transmission fluid similar to ordinary pickup trucks and vans, larger ones may in fact require multiples of those amounts. While most buses available cheaply will call for conventional oil, synthetics will still extend component life. But even at the 15 quarts a short bus might take, that can have substantial cost. A larger bus might well use twice that much. And then there's coolant; your car takes a couple of gallons, a big pickup takes three or four. A bus with a 5.9 will likely take six gallons or so, while an 8.3 might require ten gallons. The rear axle could easily require eight gallons of gear oil!
Depending on where you're going, what you're doing, and how much you're doing it, there are many potentially reasonable choices of bus. You should spend a lot of time carefully considering what makes sense before you actually lay down any money.