The back brakes only do about 30% of the braking. My car is at 90K and the rotors and pads still look good, and I am pretty sure they are original. You didn't say how many miles are on your car, and that has a lot to do with it.
If I were you, I would take the car to my mechanic and have him look at the brakes before doing anything else. If something needs replacing, ask if you can bring your own parts. Some garages are fine with that and some are not. If he is o.k. with you buying the parts, l would suggest looking at the websites for Pelican Parts, oembimmerparts and rockauto. Also, ebay has some good deals on brake parts. If this is a BMW-saavy mechanic, ask him for brand recommendations - you have a lot of good choices.