Slipping is bad! It often happens on the hiway when the debris from failing reverse clutches packs the filter.
I would service the trans, with a new filter and see if the problem continues. I too might make similar recommendations (as did the shop) if the level of clutch material was of significance (very experience driven evaluation). I would not condemn a trans on fluid alone.
The prices you quoted are a problem. There probably is no such thing as a new transmission. MB sells rebuilt units and they are much less than the price quoted. The rebuild price is a little high depending on local labor rates. We do them for about $1650 and are at 68 an hour. Most of the cost is labor about 17 hours if I recall.
Bosch Master, ASE Master, L1
33 years MB technician