My Benz is once again in commission. I replaced the alternator, which was kind of a suck-ass job, but honestly not all that bad. You just have to yank off the air filter system including the tube going to the turbocharger and you can get at it all from the top. You can also get there from the bottom, but I was doing the work on dirt and couldn't get there that way (can't put a jack stand on dirt, can't really block tires safely either.)
Basically, there's two bolts and a tensioner. You loosen the bolts until the alternator is moved by the tensioner, then you back the tensioner way out and you can weasel it out to the side. It's attached to the top bolt, and the bottom one slides right out. The sad thing is that this is only a 55A alternator, and the voltage output is pretty low at low RPMs. I've read about a hack to increase the alternator voltage on mbz.org. They have an articles section that owners of older Mercedes may find useful. The article in question pertains to a Pantera but they use the same sort of Bosch alternator that my W126 300SD does, so the technique should help me get added voltage (though not amperage) out of the thing. Also, it won't change wattage of course, since P=VI.