It sounds like it could be beyond a easy fix. I just want to
express my opinion that a new transmission will not work
right if the vacuum control or other external signals are
not correct. That may be stating the obvious, but I did
not realize it when I spent over $1,000 (on ebay, new) on
a trans for my 82' 300sd. Add to that $250 for a rebuilt
converter and $550 for installation and I have a trans
that shifts too early or too hard or a compromise between.
I like the 4spd manual conversion idea if you are able and
the adjustments are no problem. Keep in mind that the
Euro version of the 300d came with manuals (but no turbo)
and I recently saw a nice 5spd go for $2,300. Perhaps the
best plan is saving for one of these. I was quoted $1,800
for a automatic trans at MB. This does not include a torque
converter. There is also a core charge, which is refunded
upon you having a rebuildable core. I just don't earn enough
to use the word reasonable to describe this price. To me
this kind of money is reasonable for a good used car.