Has it had a timing chain, tensioner, and guide rails? If you hear chain slap - don't drive it. Has the engine had a valve job? It will need one in the future - or at least valve stem seals.
The 560 has hydraulic suspension - has it been maintained? A fluid change should be done every two years (I think that is what the book calls for).
Front suspension work will probably be needed in the future.
Does it have green coolant? Flush and fill with either MB coolant or Zerex G-05 (yellow in color).
After I bought my 420 I immediately changed all the fluids and filters. That way I knew it had been done - cheap maintenance and peace of mind.
Do a search on 117 motors and 560SEL maintenance. That will give you more info.