First, get the full warranty history and CarFAX or similar report. You can do an independent inspection if you're not fully comfortable checking the car out by yourself. Check to make sure all the recalls are done (ie HPFP, latest ECU data etc). Also, see what the maintenance schedule is indicating in the i-Drive / cluster.
Most important thing is to check condition of the turbos. Wastegate rattle is common on these cars and can be a nuisance. You can find videos on youtube of what that sounds like. Also confirm, you don't have excessive smoke coming out of your exhaust, which can be a sign of blown turbo seals. Otherwise, test drive the car thoroughly and check each and every single feature on the car