If we are talking about 1991 560 SEL's (the specific year and model the OP referenced in his first post), then if anyone finds a listing for a 1991 560 SEL that's very near perfect condition for $10k or less? PLEASE send me that ad. Strong chance that I would buy it... IMMEDIATELY.Some more for you to consider. I haven't read these ads in full, so I have no idea how good these cars are. That's for you to figure out, but as time allows, I and others will help out.
I don't think you should pay more than $10K though. And that's for a car very near perfect. (Matter of opinion; others may very well differ.)
Knowing what I know about these cars now, after doing a ton of work sorting? I couldnt recommend that anyone buy from one of those little independent lots. It's not cost effective for them to go through and sort out all (or even, most) of the things that need to be done to these cars. So while they may clean up nicely in pics, the gremlins ARE hiding.