If the 'story' checks out and the owner seems sane/engaged/honest, I'd have no concerns. Anything doesnt ring true, you should ask to have them release the service records to you to see if there is any history.
The biggest issue is price...not a lot of comps, and owners dont like taking a beating on a 'new' car. For a brand new X5 you should pay 10k under MSRP. Someone just sold their 12 with 11k miles for 47k, carmax is asking 51k for it now. Personally, unless one is saving a chunk of $$$ by buying used, saving a few k over a long term ownerships isnt worth the effort. If you are a shorter term holder, it may be worth it.
Keep it as simple as possible...but no simpler.