For E89s generally
Check via VIN which options are fitted
check service history / mot history
Buy BMW specific fault code reader and check codes of potential purchase
Check all electric functions..especially roof..operate the roof on the fob and interior buttons without the engine running
Check PDC, radio, heates screen, air con etc
Look for cambox leaks and misting around oil filter area.
Check tyres for who-flung-dung Chinese low grade tyres /inside wear /cracking
Check discs for scoring
Ideally check inside of alloy wheels for cracks /repairs
Seat bolsters for wear
For N54 specfically
Check for oil leaks from turbo area
Check gearbox for leaks if DCT
Check dampers if for misting
You can't easily check but
injectors can be an issue
coils n plugs are replaced as a matter of routine
HPFP can fail
coolant pump and themostat fail between 60k and 100k
turbos can leak / fail