16v engines are expensive to fix
8v is a solid well proven engine.
The 16v is JUST as proven a unit as the 8v 
indeed... expensive repairs compared to 8v is rubbish..
and visually outside they are kind of similar (see below) and yeah there is a 4 door 16v as poster before shows.. a mate of mine used to have one too. 3 Doors are actually rarer and desirablility between 3/4 door is debatable. also i think 4 door valvers are very common in dragon green for some reason.
(so ignore any thing BigDaveC said about valvers before....)
- 16v has obviously a much bigger engine under the bonnet with 16V in big letters on it. 8v looks like half the engine is missing..
- the 16v (and vr6) are already lower than the 8v gti stock.. but as mr '6 said.. additional lowering may suit you..
- the rear of the car will have a red 16v badge under the golf gti badge
- the grille _may_ have a red 16v badge.. some people add the gti badge too. mine came without the red one and i paid a fiver for it at a stealership
- the V5C _will_ list "golf gti 16v" not "golf gti" for valver.
- the rev counter red lines not at 5000 (8v) but at 7200 in a valver.
- as with the vr6 the 16v comes with ABS and that EDL anti-slip system that stops ya wheelspinning when pulling off (its not very good on loose ground though so be careful stomping).. its active under 25 mph. i assume this was to try and tame the beast.. but i still skid pulling away on slippy roads sometimes..
my advice.. as already stated.. look for good history, servicing etc etc.. and if you're patient you may be able to find a spec'd up one with leather, air-con, sunoof etc (like i've gotÂÂÂ

)
For me the car comes into is own on the motorway (as a GTI should).. so if you're going to test drive one try to get it on an open road to get a feel.