Guys, your advice to do a site search is good, except for one thing. He is going to be overwhelmed by the amount of information that he will find. I know, it's all there, but four hours won't even begin to sort it all out.
IMHO, the W123 and W124 platforms were good. It might be easier to determine which models to stay away from rather than try to pick a "guaranteed" winner, however.
Merc1, my experience with various cars (and this is sure to stir the pot!) is that each new BMW, MB, Ford, etc. that I bought was worse than the preceeding one. With one exception: Toyota. They kept getting better. Having said that, I found that my best solution was to find a car that suited me and stay with it. That's not to say that someone else would feel the same about the cars I own. Everyone has different needs and different likes and dislikes. Find a car that you think might be good for you and then have a reliable mechanic look it over and give you an opinion on condition.
I don't know your exact location, but if you are near Tucson, Marty Hernandez at Dieselmaster knows MBs inside and out and could give you a good report on a potential buy. I spend the summer in Alpine and winter near Green Valley and other than myself, Marty is the only one who can put a wrench on my MBs.
Good luck with your searches (both this site and for the car).