The old school bimmer lovers will tell you to buy old, and the new school bimmer lovers will...well you get the point.
Honestly, to be totally unbiased, if you're looking for reliability and to have a good commuter, buy the newest one you can get your hands on, and the best warranty for a problem free, worry free BMW experience. This goes for any car make imo. If you want to spend less and have a toy to tinker with, then buy old.
BMW...Now you're playing with power.