I went through the same process not too long ago and decided on a W124. There is no doubt that the mid 80's W123 diesels are practically indestructible; however, I decided against a diesel due to the cold winters in Canada as well as the sluggish acceleration. IMHO, the W201 is too small, while the W126 is too big. Therefore the W124 was the obvious choice for me, as it is the right size and provides a decent compromise between acceleration and fuel economy. Inexpensive W124's are few and far between, so I settled for an 87 260E with approx. 135,000 miles. The price was $CDN12,000 ($8,000US). There are a few common problems with the early W124's. If the PO has addressed them and maintained the car properly, it should last forever. Those problems include:
-worn valve stem guides and seals (relatively big $$$, the cylinder head has to be taken off to replace those); if the PO has installed the newer type guides and seals you are laughing;
-automatic climate control system (expensive and difficult ot work on);
Those problems are discussed at great length on the Forum, just do a search for any of those topics.
And a little bit of trivia. Did you know that the W124 body has a drag coefficient (Cd) of 0.30? Not bad for a fifteen year old design. Even today, very few cars can claim such a low drag.
75 FIAT 124 Spider