Same here, at least on the M103 6-cylinder the water pump job is time consuming but otherwise not too difficult. Some things you'll need:

1. Tools for removing the fan clutch (short 8mm allen head, pulley holding rod)
2. 13mm short socket and some U-joints (or there's a Snap-On swivel socket on a long extension that makes this real easy), unless yours is secured with allen bolts.
3. Short coolant hose between the thermostat housing and the head (might as well replace it while you're in there, it's a PITA to get out otherwise)
4. Be prepared to replace the belt tensioner, they often don't re-tension if they have been in the car for a few years.

Even if you have to buy everything above, you still come out ahead about $400 or so from taking it to the mechanic.