Paid $2200 for my '81 300SD. It had no maintenance records when I bought it (in fact, no paperwork of any kind other than the title). The only significant repair bill so far has been the transmission (more than I paid for the car!--I am not a DIY on anything beyond light bulbs). So barring a disaster in the near future, the cost per mile should continue to drop dramatically.
The car was driven in Maryland before I brought it to Oklahoma, only real rust is on the bottom of the front fenders. One big thing I am glad of, it has the vinyl interior, not quite as nice as leather but has worn much better than leather would have for 21 years (only a slight rubbed off spot on the driver seat back). I highly recommend trying to avoid a leather interior on an older car if you can. The right rear interior door handle needs to be bolted back onto the door, but I have left this for now as I rarely use the rear doors anyway.
Mine also has manual door locks, the only power on the car are the windows, sunroof and right side mirror. Being a Jaguar owner, I can attest to the value of avoiding power locks as cars age.
The A/C was converted to R-114 by a shadetree mechanic, but my mechanic said all he had to do was tighten the ground wire and he feels the system will work fine, though he did put an "unknown" material label for the referigerant, which might be a problem in some states (not in Oklahoma).
My main criteria was to have a car at an inexepensive price that looks and drives nicely, and so far the 300SD has been all that. Oh, I also switched out the mirrors on the sunvisors so my wife would have a clear one, looks like only the passenger vanity mirror ever was used by the previous owners, a trend I suspect will continue with me. And that center sun visor, what a great idea! Too bad it never caught on.
I also have a theory which many will find heretical but I will share anyway: With these older well-built cars, find one with a good interior, a good exterior, and be ready to put a little money into engine and mechanical. From all the quotes I have gotten, the engine and drivetrain always seem cheaper to repair than the interior if new parts are required, and body work is expensive too (interior part prices border on the ridiculous).