This is just info from my experiences...
I bought a 1987 560SEL with about 155,000 a little over a year ago. The first owner had it until a year before I bought it and maintained it to a "T". The second owner did NOTHING aside from one oil change for the year that he owned it. I had it checked over by an independent shop before buying it and they quoted me all the work that was needed. They charged me $75.00 for a full look over, and found a few things (leaking PS hose, accumulators needed replacing, front and rear brake pads, valve seals, sunroof angles, and a few other very minor issues. After working out the price with the seller based on these problems I bought it. Since then (I know it's only been one year) the only problems I've had is that the rear passenger side window screwed up (small plastic piece-cost: $6.00, but lots of work and grease to replace). I've driven to and from L.A. three times (I live in the bay area) and I've put just over 20,000 miles on it in the last year. Like Paul C. said, these cars will run forever as long as they are maintained well. Even if everything checks out, be prepared to spend some cash if needed, it's a great car, and for that reason, parts aren't cheap. In a short time you will be able to do some work on your own, saving yourself lots of cash. That is my rant...good luck!