You'll need one with the sport package AND 18" wheels or you don't get the auxillery oil cooler. And with no cooler, that's a car you don't want. A 335xi with manual, sport, and 18" wheels is a fucking unicorn. Very rare, highly desirable. You can however find them with steptronic and the oil cooler.
The cars have 2 weak points and they're both related to the direct injection system. The HPFP tends to fail, but new revision pumps are more reliable. BMW has a 10 year 120k mile warranty on the HPFP and injectors. 2nd is carbon buildup. It's not nearly as bad as Audis of a few years ago, but it happens.
The cars will do 400whp with boltons, more with meth, and almost 500 with boltons+upgraded turbos+meth.
And get you 30mpg+ cruising 75 on the freeway.
Edit: Also, early N54 engines are rumored to use full forged internals.
Only difference between 07-08 is addition of CANbus to the OBD2 port, 09 for a facelift (LCI), 11+ switches to twin scroll, single turbo N55 engine.