My all time hit list.
Interesting topic as I agree that the most fun is to find used deals on older MB's. The interesting question is which ones?
I have owned the following cars:
I believe the best (cheapest) deal overall is the 300sdl from -86/87. They are extremely reliable, cheap to drive and luxurios. Very comfortable drive and the 126 chassis is both safe and handles as well as most modern cars.
If a diesel doesn't suit you but you still seek cheap reliable transportation, it is very hard to beat the 300e from 91/92.
They have the very reliable straight 6 (-93 got the new 24valve head, which I believe tends to have leaking head gaskets) with 177 hp. It is quite modern chassis and is quite agile as well.
If you are ready to trade some reliability with speed but still want the old school MB's I believe a e320 coupe from -93/-94 is a very good buy. The model is quite underrated and can be very good bargains. Of cause the 4 doors counterpart are quite agile as well. A used 400e can be very surprising to most people.
If you really want performance but still want to be thrifty with repairs I believe that either the E500 or a SL500 from the -92 to -94 are great cars, but they still demand good money.
A great alternative are the 560's from -86 to -91. They may not be as fast but they are great rides and do feel quite powerful when they get rolling. I just got a 560sel from -89 and the drive is a thrill. Ofcause you pay at the gas pump but I believe in terms of comfort/speed/reliability/depreciation, these top of the line 126 bodies are quite hard to beat. Whether the 560 sec's are future collectables I am not sure?
The prime collectables are the 500E and the E320 cabriolet.
And ofcause don't forget the 560sl, which holds their value incredible well.