Did they try a valvebody gasket and retorque?
If the trans does not shift beyond second it has a specific problem that can (and should) be identified to either mechanical or valve body. This is done by pulling the valve body and exercising the tractive components (bands and clutch packs) with air pressure and if functioning a replacement V/B gasket and retorque often fixes the whole matter. If not it is reasonalble to assume that the V/B is defective. Knowing how to do this testing tells whether you have someone capable of advising you. You probably will find the best MB trans rebuilders in independent MB specialists shops. Trans shops don't do much diagnosing and MB dealers often don't do much more than install MB rebuilds.
Making this decision is important as it definitely affects the outcome and how one should proceed. If rebuilding is the answer and the V/B is the problem you could wind up buying a V/B after the rebuild. If I have a question about the V/B I believe a MB rebuilt becomes a good way or a used trans.
Bosch Master, ASE Master, L1
33 years MB technician