I also have an '86 300E and I found the alternator weak. Without load it charges by about 13.75 volts, however if I have the rear window defog, the heater, and the full headlight set on inc. foglights the charge drops down just above 12 volts. MB changed the alternator for a stonger one in '90 from 70A to 80A. The 400 and 500 has a 110A alternator. So go for higher amps if you can.
Incidentally my alternator was rebuilt this Tuesday (the bearing was completely shot) and gives me the same output readings as before so I guess it is not strong enough for the electrical system.

Alex