Well, if you would like it for track day use but also road use and would like to end up spending quite a bit on it, I would recommend buying a 3SGTE and having that put in, R32 GTR Skyline front seats are comfortable as you like, hold you tight round the corners and are quite cheap, KEY!S Racing do some excellant sports steering wheels and you can never go wrong with an Omex 400 ECU and a HKS Catback Stainless-Steel exhaust system.
However, you would have to go through everything else to get approval for the new, bigger engine to go in. But yeah, it's easy to give it a bit more power just basically by replacing a lot of the engine internals with newer versions of the exact same parts. But I would have to say the most important modification it should have is an Induction Kit. It gives it around a 10-50bhp increase (depending on the make, the type and if it has a heat shield, full pipe, etc.) and with one the car ends up a lot better on fuel.
Probably the best thing to do is give TRD (Toyota Racing Development) a call and talk to someone there about it, they would know a lot more what to do with it than us. You can do a lot of good things whatever your price range is and whatever you want to do with it.