I've retrofitted cruise (is relatively easy once you source the right parts, no new wiring but requires coding), and added heated/memory/msport seats (which does require wiring depending upon your original spec, and coding). the latter took me much longer than I hoped because of a lack of full information on the web and not many people have done it on the E89. I've posted my findings on here so just look for my previous posts if you're interested. Most people would just buy the car with those items fitted, but I paid a lot less for the car originally (because it was less desirable), and enjoy working on the car.
There's more than one issue on the doors freezing up - on my car, its not the rubber around the door causing the issue - its actually stuck in the door somewhere and takes a while to defrost with hot water getting the car heating on. I've yet to work out what is actually causing that. I can eventually get the door open but the door window can sometimes remain almost fully up and doesn't retract enough to let me close the door as it fouls on the roof. Its not my daily ride so it doesn't affect me that often, but when it does its a PITA!