Go for the 85 300SD. I owned a 1987 300SDL, which is the diesel version of the 420 sel. At 150kmiles, I started to have lots of failures (a/c, front end, drive line problems, etc). The engine was strong and smooth, but the chassis and driveline was going away.
I believe the earlier years are more reliable. My 240 has 224kmiles and has never had the failures I had at 150kmiles on the 300 sdl. I don't know why, but my guess is that MB started to cheapen (is that a word?) their product in the mid 80's to compete with the top line Japanese cars.
Of course, this is only a single data point, and might not be true overall. However, I know what I would pick given your choice...the 85.