You need part 15 (carrier, outside door handle) on the diagram from RealOEM.com. Guaranteed.
Common failure part on the X5. 
No need to change locks or keys or anything, the carrier is just the 'guts' of the door handle assembly that actuates a cable that pops the latch to open the door.
There should be some previous posts on how to do this job, its not terribly complicated, hardest part is finding some of the screws that need to be removed to get the door panel off. Check the window regulator articles on the homepage for door panel removal. Once the panel is off, you can see the nuts that hold the carrier assembly in place. Oh, (very important) you do have to pull the black oval shaped plastic plug (part 3 in the diagram) out of the side of the door (above the latch) to access one of the nuts that holds the carrier in place.
I helped out igosal with this project a couple of months ago. Its not too bad if you have the basic hand tools and don't mind spending a couple of hours doing the job. Or pay the dealer 350 bucks!
