The original question is hard to answer. The easiest answer would be to say that we charge 6 hours labor to R&R a tranny and 3 to reaseal the front pump and external housings.
The pump is bolted from the back side of the forward housing, which requires disassembly. If you are scared that rebuilding is risky then you should be scared to do this work. Other than the valve body the mechanicals inside the tranny start where you are at. It doesn't get much harder to replace the Reverse clutch at that point and then the next and the next etc.
If the tranny has been rebuilt within a noticable time then I would worry that the front pump bushing is worn. It would be wise to have an experienced eye to look at the problem.
Bosch Master, ASE Master, L1
33 years MB technician