Some MB mechanics say you should never rebuild or buy rebuilt MB calipers.
It's interesting that Caliber Mercedes in Anaheim, Calif. sells rebuilt units that they claim Mercedes guarantees. Their price for these rebuilt units in the case of a 126 car was WAY cheaper than brand new ones. I cannot say what they would get for rebuilts for your car.
You might give them a call. If you do indeed need to replace calipers, it might be better to buy something that MB stands beninds vs. attempting the rebuild yourself. Bad calipers can cost you or others their lives.