Amazingly, all the items you mentioned could be specified as heated from the factory. The door mirrors, driver and passenger door locks, and windscreen washer nozzle heating were all part of a "winter pack", but I think that option is rare on cars in Europe, I guess it was an expensive factory option.
I've heard that the later (90, 91, 92) E30s have the full electrical loom installed regardless of options selected, so retrofitting the heated mirrors, nozzles and door locks isn't too tricky. Thats also why the wiring for the door tweeters is there whether the premium sound system was installed or not.
Don't know much about the heated seats, though there were definitely an option, I see hoevesruperd is lucky enough to have them though!