Look under Resources at the top right and there should be a DIY link on how to change out the waterpump. The trickiest part may be getting the fan clutch off because you have very little room and need to hold the pulley from a hole in the back of it. Once you do that, it is only two bolts to take it out and put it in. A local repair shop wanted to charge me $800.00 just in labor to do it (based on Mercedes procedure recommendation). I did it for the cost of the water pump and a few hoses.

Here's the DIY Link:

http://www.peachparts.com/Wikka/M103WaterPump