I would avoid the accident car. $20k is not minor even on an X6M. That did not happen to be a small bump in a parking lot, that was a proper hit! They are BSing you about the extent of the damage. If it was suspension and the like, there's a likely possibility the car will never drive straight.
I would not buy no matter how good the deal was, especially on a high performance car like this. But I would look for another X6M though