The post-refresh 300Es would be your best bet, and anything from 1990-1992 would be preferred. Only major issue is the head gasket, which I had to deal with on my wife's 1992 300TE at around 100,000 miles.

Just about any MB motor is going to have valve guide/seal issues at some point. If this hasn't been done or isn't (yet an issue), the time to do it is when (not if, but when) the head gasket goes and the head is off the car.

The seals can be done with the head still on the car if that is the only issue there. Only problem with that, is that it's preferable to do guides and seals at the same time just to take care of the whole problem.

Good luck!

Cheers,
Gerry