I have had two 560SL's for a total of 10 years. I bought one in 6/99 with 30K and sold it 7/06 with 80K.
http://pg.photos.yahoo.com/ph/lynnsorensen1/slideshow?.dir=/bbfcscd&.src=ph
Pay attention to the following.
1) Has the Behr radiator been replaced. The upper plastic kneck is known to crack and can cost $750.00. If the radiator has been relpaced. Was it with a Behr? No aftermarket, please.
2) Make sure that the center A/C vents are functioning. The center vent accuator's are known to fail. The inner diaphragms are made of rubber and get old due to environmental factors. I had mine repaired without removing the dash by a knwlegeable INDY for $450.00.
3) Check the rear main seal for leaks. I understand that all older MB leak. I did not fix mine because it only leaked a couple of occasional drops to a garage carpet and the cost of the repair.
4) Sub-frame and motor mounts have already been addressed.
Bottom line - plastic and rubber parts age due to environmental factors such as heat and Ozone. There is really not much that can be done to alleveate this except routine maintainence and $$$$$'s spent. Check to see if the person that owns the car can show you a list of maintainence that includes date, mileage, and description of what was done at repair. If they don't have the list and receipts then buyer beware.