i've also got a GS and would deffo recommend the ITG induction kit, cheap but lovely throaty roar when you hit 4000 revs.
but as far as mods would go i would firstly recommend a decent set of alloys and then lowering it, without the bodykit the car still looks the business with a nice set of 17 or 18's.
as far as other mods go it's all a personal preference really, i started with alloys then bodykit, a decent exhaust system, 4" can each side, then lowered it, induction kit, and the exterior is finished, i started on the interior then, all the usual dash stuff (some more new stuff on the way) deep dial rings, door handle cups, steering wheel, that sort of stuff, then the big one, the cream leather, which apart from the body kit was one of the best buys i've had to date, completely transforms the car.
all i need now is some decent ICE, and then there's not much left to do, oh apart from dropping in a nice V6 lump