21s do fill the wheel wells better. Go for it. Which brand wheels.
F10 - M5 brake parts below. You'll have to contact LTBMW on the custom caliper brackets. Also, some realized that it requires front fender flares (front M5 conversion) so the wheels don't stick out. I don't know if your F07 requires fender flares or not.
Left Caliper 34117845747 $767.66
Right Caliper 34117845748 $767.66
Left Rotor 34112284101 $394.57
Right Rotor 34112284102 $394.57
Pads 34112284369 $346.10
http://f10.5post.com/forums/showthread.php?t=702414
Those are M5 calipers, painted classic Brembo color. Technically, M5 calipers are made by Brembo.