I'd for a CPO car. (Presuming you live in the U.S. - I do think BMW offers similar programs in other parts of world).

If you do consider buying it, take it to a dealer and have the dealer look up the service history (they can run the key and see what was performed on the vehicle) & run a carfax.

Someone posted a thread a couple days ago about buying a 60k mile X5 and got lots of responses... looking there would be a good start.

Good luck with your purchase regardless.