The only turbo kit I know is the VF Engineering that seems to be sold by just about every parts distributor. Its apparently (not tested) a very complete kit that should be plug and play.

If I may recommend you try one thing before going to a turbo kit or supercharger : find a 4 wheel dyno, take the x5 as it is and make sure you've got all of the original horsepower or close enough. I've known a friend wanting more power simply because his car needed a good and complete service to win back 20% lost power.