On one of the tags in engine bay (passenger side if I remember right),will show as GF8D5ED if it is a version 3 STi. If earlier, it will be same code as a wrx (GF8D58D). Easier to know which model in 4 door cars.
Going by the pics in the ad, looks like a version 3.
Version 3 didn't have the ICS (water spray). Check for the red paint on inlet manifold (this can be done on wrx's too so be careful). Should have carbon sti strut brace, quickshift gears, carbon speedo face, quick rack steering, 4 pot front brakes, lightweight bonnet. Hope this helps.
Saw your other post asking about boost pressure (had to rush out so couldn't post on it). Standard boost is 0.8 bar and they shouldn't really go above 1 bar. Personally I wouldn't even go over 0.9 on them!! Hope this helps