I recently acquired a 91 300 SEL (140,000) from a very reputable private source. - with six inches of dealer documentation to back it up. Here are a few anecdotes - from a newbie:

- Door hinges on the 126 are notorious - something about the device which keeps the door open after you unlatch it. I've now replaced two of them (fortunately my mechanic is quite innovative, and not interested in buying stock parts.)

- Window motor units - the scissors inside the door. Check them all for any noise - which means they are about to give up.

- Unfortunately, the PO's dealer was not absolutely truthful on this point - which I just paid 2,200 to rectify. There is a filter in this transmission (inside the transmission casing) which requires periodic changing. Documentation showed 2 replacements of this filter prior to my purchase. My trans gave out just 3 months after purchase. You should check 3 points: 1) find out the standard replacement interval on this filter, and confirm it's been done. 2) smell the transmission fluid - if it doesn't smell like perfume - beware. 3) if there is any delay in shifting - even when engine is cold - beware.
My trans was replaced by a benevolent trans shop = expect a 3-4,500 bill if you don't have a friend in the trans business.