My two cents...
I'd recommend an '83 240D.
This was the last year for the 240, so they came equipped with sunroofs and other extra goodies as standard. All have auto transmissions, I believe, but on the W123 cars, I think you will find the power curve acceptable, once you get used to it.
Parts for the older models are now getting scarce, but the 240 W123-body is still plentiful, easy to get parts and service for, and will not look dated for a long time.
A key...THE key...to keeping an MB diesel running forever is oil changes every 3,000 miles. Try and document how often your potential car has been serviced. Also, your oil pressure gauge should peg at the top or "3" mark when the engine is warmed up and running (not idling).
Best of luck.