As mentioned above, I have no personal experience in this area as my X5 has the ZF transmission which I haven’t had any issues with however; having read many posts about various failures with X5’s since purchasing mine a few months ago, I personally wouldn’t consider one of the pre-facelift (5 speed auto) variants unless it was a manual.
The auto in both the 5 and 6 speed is extremely expensive to repair and that’s pretty much the only fear I have with my vehicle as I reckon I can tackle most other repairs myself but would never attempt a transmission rebuild. Failures on the ZF aren’t un-heard of either.
I would also look at whether this vehicle has a tow bar already fitted, and if so, if you can find out what it’s been towing and how often.
If it’s done a fair bit of towing anything substantial it would add further weight to perhaps giving it a miss.