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..
2008 E90 335i LCI M Sport / Manual / Black / Red Leather / Avon ZZ5 / VMR710
Mods: JB4 G5 ISO / BMS Back End Flash / BMS DCI / AR Design DPs / ETS FMIC / Meth Injection / Spec Stage 2+ Clutch / BMS OCC / Cool Carbon Brake Pads / RB PCV Valve / Forge DVs / M3 Control Arms / Bilstein B4s / Eibach Pro Springs / Eisenmann Race
Wish List: LSD