Leather was available in all years, but the early ('86-'89?) leather was a bit more durable/stiff and the padding not as good. Look for the perforated leather strips down the middle.
Not plug & play, I am currently putting '94 seats in my '87. The memory module is different, I have gone with this thread from Johnscars:
Swapped seats from a E320 into my 87 300TD
Says the driver's seat needs to be swapped with the old motor/base, passenger seat is plug-n-play.