My experience has shown all the 2 valve V8s to have similar wear characteristics. Notable changes were in the valve guide oil seal style - the last change I know of came in 83-84 (before 420), the valve guides/valve stem diameter - came with the aluminum motors - 1980-81.
I looked up cam numbers and they have been the same since the 420 came out except in the european version where the number currently in use for US models changes. The US numbers are 117 051 08 01 and 09 01; these numbers chnge to 20 01 and 21 01 after engine number 965: 048805.
Most changes to the car were cosmetic/trim from 86-91 although the car did enter onboard diagnostics in 1988. The A/C controlled the aux fans and was smarter from 88.
Its probably more important to find a nice example of any of those years than to get a particular one. I would go for after 88- though as a start.
Bosch Master, ASE Master, L1
33 years MB technician