The used one sounds good to me. Rebuilding a MB differential is not something one should invest in learning. It requires a number of special tools and techniques not often used.
Bearings are easily available from a good bearing supplier. The generic numbers are right on the bearings. Seals probably can be had from fastlane but the are cheap enough that the dealer works. The gears are only going to be available from MB.
Bosch Master, ASE Master, L1
33 years MB technician