+1 to Dackel's response.
There are only two sensors. You only need to change them if they become damaged. Actually, damaged sensors seems pretty common from what I've read/seen. They're like $15 or $20 each and usually always in stock at dealerships, though, so it's not a big deal. Sometimes they get damaged during the install or something. I've seen OEM ones at various online retailers, too, but I haven't seen any aftermarket sensors.
Rotors don't wear nearly as fast as the brake pads, so older front rotors shouldn't really matter.
The pads are $285 with free shipping from
Bavarian Auto, and the same price from
EAS (but not sure about the shipping).
If you're not sure about the sensors, you can probably get them pretty cheap if you just PM Dan@UnitedBMW here on the forum.
Also, how are you liking the performance rotors? I'm about to order some from Dan. I know they probably don't offer much of a performance benefit in everyday life, but I'm just curious what you think.