Shawn,
I replaced my clutch and water pump this last weekend. I do not believe that you can rebuild the pump. I have the shorter version and it can't be rebuilt.
I bought an OEM pump from MB, not an aftermarket. The pump that I replaced, although still working fine, was an aftermarket. Comparing the two, I noticed that the impeller on the MB pump extended into the housing about 2-3mm more than the aftermarket pump, yielding better efficiency. I recommend using only original MB for the pump.
The system now seems to cool much better.
The next step is to replace or recalibrate my gauge, which I confirmed is reading approximately 18-20'F higher than actual.
While you have everything else off the front of the engine, now is the time to replace the front seal.
Good luck.
Regards,
Stan