Unfortunately what you really need to do is have someone like me or Aaron do the things on Aarons list on any old MB you want to use for a while.
I can tell more in a test drive, walk around, and quick blow-by test due to my experience than all the looking or reading that you can ever do. Drive (and oversee the work on) ten of these cars a day for twenty five years gives experience that can't be learned from a book.
You will need a good shop for some of your work on an old MB (unless you got more time than money and are in training). If you don't know who that place will be then I would use the pre-purchase inspection to inspect both the shop and the car. They are both equally necessary.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician