Lothar,
I am in a similar situation with a slipping 94 E320 transmission. There are at least 4 paths to choose from.
1) rebuild existing transmission. The most sensible choice
2) install a 6 speed manual transmission. I believe you want one from a 4 cylinder car like slk230 to mate with M104. The 6 speed has a low (high numerical) first gear which is a good match for the 1994 2.65 rear differential.
3a) install the C280 driveline. Engine, 722.6 transmission and 3.07 rear axle. Changing the axle ratio will improve acceleration so the 2.8 engine may be fine.
3b) like 3a but use the 3.2 engine with the necessary parts from the 2.8
4) 5speed manual transmission.. All parts w124 series so minimal or no fabrication. 3.07 differential probably best
I'm thinking of 2) or 3) for my 94 wagon using a 97 E320 as a donor. My existing transmission only slips occasionally so repair is not urgent for me. Let me know what you choose

Thanks to everyone in this thread for all the OEM vs aftermarket discussion. It's been a great help to me