if you are in the market for a used X3 things to check:
1. rattles - check doors and pillars for rattles.
2. suspension - especially back seat. the suspension can be very very bad in the back with every bump, rut or imperfection in road causing massive upheaval to the car.
3. transmission - don't be afraid to test drive as you would normally drive. the hesitation you feel is not just you being cautious. it's real. go up a hill and the car will downshift and then.....take forever to upshift....even when you think it should shift to a higher gear it may not.
we had one for 3 long years and the best day of our lives was when we turned it back in at the end of the lease. if after you test the above you still like it, then best of luck !