There are a lot of threads about this - what 140 is it?
All early 140's had front suspension problems because of weight. Evaporator cores are a major headache and expense. Most 300SD's are trouble waiting to happen - some never seem to have problems - engine wise. Door closing assists seem to be a major headache. Wiring harnesses on the '95 and some '96s were biodegrabable - too much so.
The 600SEL/S600s are very expensive to maintain. A search will turn up more info for you.