We have carried out similar projects here for our customers(eg.M3 engine in 316i!!).Before you buy the M5 engine,do the following:-
1.Check out the engine mountings on your x5 and on the M5 engine to see if you can adapt them.
2.Check out the gearbox in your X5 and take note of any alterations that have to be made and if these are possible(I am assuming that you are not buying the M5 engine with a gearbox!).
3.If you are happy with 1,2,then ensure that you ONLY buy the M5 engine with the ECU and the respective wiring.
4.Now comes the hard work,but if you managed to get this far then altering air-filters (the M5 has 2!) and exhaust systems on either side,and connecting up all the wiring will be satisfying work in the long run.

You might also consider getting the M5 engine with gearbox but perhaps the gear ratios are not too well suited to the X5.
Good luck...If I can be of further help let me know!