First off welcome.
I had a mk3 16v until recently and did a lot of research into what mods work best.
I had stainless exhaust, pipercross panel filter, weitec coilovers, BBS RX2 alloys, brembo discs/pagid FR pads up front, standard rear discs and pads. Was a great little car.
Brakes/suspension/wheels - Standard discs with Pagid fast road pads up front and possibly 288mm upgrade if yours is a pre 96 car. Decent suspension kit (Weitec or Eibach) 50f/30r to even out the ride (mk3's sit higher at the front). 16" alloys big enough without compromising ride/handling.
Engine - Milltek cat back exhaust, decat or sports cat, drilled airbox with panel filter, remap. Most/all of the engine stuff can be done at AMD near Bicester. Should see 180 with all of that stuff combined.
That little lot will probably set you back just over a grand i would say and you'll have a very nice motor indeed that goes/stops/handles well.
Just my opinion.
Stomper
