90k miles on a 2011 is not "high mileage", that is low mileage.
See if there are any maintenance records and what your PPI says.
If it checks out and the price it right, go for it!
The main things that you will need to contend with in the next 10+k miles, if they havent been addressed already are in this thread:
https://x3.xbimmers.com/forums/showthread.php?t=1730393
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Current: 2020 X5 x40i M-Sport, Arctic Grey Metallic / Ivory White, Executive Package, Luggage Compartment Package, Integral Active Steering, Tow Package, Parking Assist Package, Driving Assist Package, M-Sport brakes, Tux Mats