You're gonna have to find out how wide those wheels are before you can make an intelligent decision on what tire is best. I don't know whether the measurement is out-to-out at the rim or if it's measured inside.
Might be a good idea to get Tire Rack on the phone and ask them which it is. Then you can get the measurement and call 'em back and go from there. Of course, you wanna try to stay as close to the outside diameter of the tires that were originally on the car. Do you know if it came with 18's or 19's?
I'm guessing you've already seen this, but in case you haven't:
*BMW equips your vehicle with more than one possible size during production.
Please select the size installed on your vehicle from the choices below.
Original Equipment Front Size 1: 225/45-18
Original Equipment Rear Size 1: 255/40-18
Original Equipment Front Size 2: 225/40-19
Original Equipment Rear Size 2: 255/35-19
Sizes To Fit Your Vehicle