I had a play with an Audi R8 this morning from 70-130--- erm KPH (without meth). I had to short shift to stop myself getting too close...
N54 with £1k of mods is a weapon which nothing short of an M5/6 in the current BMW line up can match.
To answer your question on what to look for, in this order:
1) Wastegate Rattle
2) Rough Idle/misfiring
3) Problem starting
I would (and did) buy a lower tax bracket one if possible iirc 57 plate onwards..
Car: 2010 Jaguar XFR (Cat-back exhaust)
Gone but not forgotten: Highly Modified E90 335i LCI M Sport Manual (circa 480BHP/500lb ft)