do it yourself, its so simple, why pay anyone else to do it ? Just make sure you put them on tight
If you've never changed a wheel before, you may want to read on how to do it first, its very simple, but if you've never done it you may miss something that would make your life easier.
Just a quick run down on what to do:
1) Get yourself a jack, also a wrench/breaker bar will make your life 10x easier.. These wrenches made for cars are long with a nice grip, it GREATLY reduces the amount of strength needed to loosen/tighten the lugs.
2) While car is on the ground, loosen all 5 lugs (dont take them out, just loosen them!). The reason for doing this, is that they are held on real tight, and you don't want to be banging on a wrench while the car is off the ground.
3) Now, raise the car off the ground with the jack, the wheel should not be on the ground at all.
4) Now fully remove the lugs, be careful as the wheel will fall off the hub easily.
5) Take the wheel off the car.
6) Inspect the hub and make sure there is no extra rust or residue.. give it a quick wipe and try to make the surface even and clean. (there will likely be rust and discolorization)
7) Put the new wheel on and put in 2 lugs at opposite sides of the wheels. Make sure you put them at opposite sides and tighten them up fairly tight. Make sure that as you are tightening the lugs, the wheel stays perfectly flat on the hub surface. Sometimes when tightening the wheel can jar in at a certain angle which would be very bad. So after you tighten the 2 lugs, spin the wheel to make sure it looks even against the hub at all points (360degrees)
9) Put on the rest of the lugs in the same fashion (going from opposite sides), make them all semi-tight.
10) Lower the car to the ground.
11) Now fully tighten the lugs, I believe BMW spec is around 88-90 ft-lbs of torque