welcome to the forum!
best performance per pound mod is a re-map for around £300 you can expect almost and extra 33% in power, another cheap popular mod is an induction kit / air filter, after this you're looking at exhaust and ultimatly bigger turbo's depending on your budget.
a cheap mod for the 1.8T is the big brake upgrade 288mm => 312mm this involves changing the discs / pads and pad carrier
cosmetic mods wise, most people opt to change the wood trim to either the R32 / anniversary aluminium or paint it their selves, pedals either TT / anniversary / R-line are a straight swap and wont break the bank, front arm rest if you don't already have one.
more expensive mods are colour coding of valances, alloys (most people go for 18"'s although the ride does suffer) if you like things big then you can get 19" 's check out Liz's pics.
have a read through
sub £100 modsi'll warn you though, once you start modding expect your bank balance to suffer
