dosoakies
You are in Calif, right? Well, just let me know how much you want for that car, swapping a different trans is likely to bring a lot of surprises (pretty car it is!) and you will not be happy! I could drive it home at 35 MPH if you are nearby and take it off your hands.
All kidding aside, find a good mechanic and present them with the trans part numbers for a potential swap, there are some who have done this and can tell from experience what the issues are.
I know a shop in the Bay Area that can convert it to a stick shift, but the cost will likely equal or exceed having the existing 4-spd automatic rebuilt. If you would like a tel# contact me by PM.
Dieseldiehard
1971 220 (gas) 4-spd manual 102600
1979 300TD w/ ’85 turbo engine 287690
1983 300D 234200
1985 300TD 202,820