I bought an 87 420 SEL in August. I figured that these 15 year old cars are not exactly cheap to maintain. My brother drove MB's for many years so I knew. There are many rubber bushings that deteriorate with age and need regular maintenance.
The best advice is "have it inspected". Is the transmission leaking? Is AC working, etc.
Chances are you will not get much maintenance records but everybody will say it got good maintenance.
Rather than repeat all the items, do a search with "560 W126"
and enjoy reading.