I would check service history with dealership (they can pull it up through VIN). If it's up to date, it should be fine. These are some notes I can think of...
Things known to have failed on the N54 motors are HPFP (most have been resolved through the recalls), injectors, and coil packs.
The turbo wastegate rattle can be annoying but it doesn't affect performance... The exhaust butterfly valve can rattle as well. Make sure the turbo's aren't shot (sounds like a siren).
Electric water pumps on all the e9x series seem to be a vulnerability. Some seem to fail at 50k, some at 80k, and some well past 120k (miles). Its like a crapshoot from what I've noticed so just keep that in mind.
Make sure that the software level is up to date or it might have the "lag" throttle response problem.
Everything else is mostly maintenance items...
If buying used, check to see if car has been chipped / tuned if that sorta thing bothers you.