Marcb,
I'd say it's a medium DIY job. It's not simple but nothing like pulling the head, for example. No special tools are required. Draining some coolant and pulling the upper radiator hose might make some extra room, ditto pulling the fan clutch & shroud (actually I think the fan MUST come out). I think the belt tensioner can stay in place. That reminds me, look at the tensioner pulleys... if they are not PERFECTLY parallel to the engine block, you need to replace that ASAP as well. this is another common failure. If the pulleys are "cocked", the bearing is shot, and if let go it can eventually tear up the timing case (very expensive, nasty job to fix.)
About the vac pump. If your engine number is something like 600 or below, you should remove the "basket" behind the pump (per the instructions) and use the "dam" gasket supplied with the pump. If you have engine 600 or higher (which is much more likely), ignore the instructions and leave the basket installed, and do NOT use the dam gasket. Use the "outline" gasket without the dam. If the new pump doesn't come with a "non-dam" gasket, order one from the dealer ($5-$10). The basket doesn't hurt anything and can prevent an exploded pump from taking out the timing chain (which we all hope never happens). And this way you can't lose it in your garage (if you pull the basket, keep it - it's needed if you ever R&R the injection pump). Oh yeah - don't put any sealer on the gaskets. They usually are a coated metal gasket. Just clean the surfaces VERY VERY well, and install it all dry. Don't overtighten the bolts either!
More details are in this PDF file (you'll need to scroll down to the vac pump section) :
http://www.meimann.com/docs/mercedes/OM603_modifications.pdf
Have fun!
