Well are you going down the turbo route or N/A route? They're both very different and will have different requirements... bang for buck is obviously turbo route as a lot of the members have done here.
Brakes -
1.6L N14 or N15 clappers bolt on, but u will need some 1mm washers to clear. You can also use the SSS or Pulsar GTI-R brakes, they are bigger and better, but a bit front heavy, so depends on what power you are chasing, for an N/A I'd say the normal 1.6 brakes are sufficient. Use slotted discs, dot 4 or 5.1 fluids, braided lines and more aggressive pads and it should be a massive brake upgrade from standard.
Seat-
Make sure you get lower seat rails too! You can find 1-piece bucket seats from $200-$1000+ (no brand cheapest, sparco and the like in the middle, recaro or bride the most exxy!)
ECU -
A lot of people here recommend GReddy e-manage piggy back ECU, as it gives you control over a lot of things that other piggybacks (such as Apexi AFC/SAFC) don't. Of course, the best is to get a full standalone ecu, but we are looking at a couple of Gs...
Quads -
we'll it's pretty expensive to do, but if you're looking to get the most out of an N/A application, you want it... and a full ECU. If it's not in the budget, do the GA15 throttle body (bolt on, but need bigger injectors) custom air chamber and pod filter with a nice cold air feed, a step up form this would be getting your own custom intake plenum and SR20 throttle body.
Cams -
Well not sure what there is out there besides Tomei cams (256 degree) which is pretty mild, and Nismo ones which are a touch more tougher (ask turbo march rolling shell). I'm sure you can get them remolded to your own more aggressive profile from somewhere else, provided you have donor cams to give.
Tires -
Semi-slick for the track. Depends on what size wheels you plan to run, I use 14s and find Toyo R888 a good compromise for street and track. Yokohama AO48 is also good.
if it's a dedicated track car, lighten it as much as possible, remove trim, back seats, spare tire, air con etc... it will take a **** load off believe me. best modification to have on the track, is your suspension and tires addressed.
