What's wrong with rebuilding your own pump? They're pretty easy.
Power steering pumps are kind of a funny thing to rebuild. The main problem with them, other than front seal leaks, is wear of the cam pack or internal rollers, depending on the model. A production rebuilder will buy 100 cores (failed units), take them apart and assemble as many good ones from the pile of parts as they can.
For high volume sellers, Ford, Saginaw etc, after market cam packs are available. I doubt if they are for the Benz.
If the front seal leaks the seal can be replaced. It's probably nothing fancy, usually just a double lip seal, and a good bearing supply outfit can find it for you. Same for any O rings. If the shaft has a groove in it where the seal rides you have two choices, try to install the seal (or use one with a different wall height) so the seal rides on a different part of the shaft, or take the shaft to a place that repairs crankshafts. They can flash chrome it quite reasonably.
If you look you will probably find a good local rebuilder. Some will offer a warranty that includes removal and re-installation if the unit fails. They're the best.
These guys
http://www.autosuppliers.co.uk/index.htm have distribution in the US and can get the parts you need.