Ok, What did I just get myself into.......
 

Just put a deposit on a 91 300TE yesterday......my first gas
MB
Teal/petrol over quartz, body and interior is in VG condition.
186k and seems to have been maintained fairly well. I got it
from a "flipper" who purchased it @ auction. I was impressed
during the test drive with the exception of a wheel bearing
in the LR.....that is being replaced before I pick up.

What do I have to look forward to, what should I be looking for?

Bearing should be pretty easy to do....

Make SURE reverse is not slow to engage!!! If it is, walk away and forget the deposit. You'll need a new or rebuilt transmission in short.

Only if you need reverse to work. :P Over 20k miles without it so far.

Just the trans?????? no other issues?

Since you currently have a W123 "TD", then I am assuming that you would know to check the self leveling system.