There are lots of entries on this month's board that can give you some ideas of the problems. The system is vacuum activated, and after 15 years the vacuum motor diaphragms degrade. Some of them are in places that you can barely see without removing the dash. I guess you could just make sure the system works well in all modes on your prospective purchase. If it was me and I wanted a daily driver that wouldn't be a part-time hobby I would go with a 3 or 5 series if you are set on a BMW.