best thing is to do your research. theres more than enough informaiton here and other supra forums to give you an idea of power achievable with all types of set ups and turbo conversions.
the bottom end NA or TT will hold enough power to suit your typical needs up to 1000bhp. the main difference, is what supporting modifications your need to do this. with the NA, your spend alot more money making power you could with alot less on the TT, just because its designed for more power so its easier to reach. a good example of this, is injectors, the NA will need an upgrade to reach 400bhp where as the TT can hit 400 with stock injectors.
the whifbitz kit does look nice, quality parts and stuff you won't need to buy twice. i wish i'd gone that route, as i collected the parts myself. im ready to fit my kit in the next couple of weeks, i already regret not purchashing whiffins kit as i probably didn't save much money and got alot of second hand parts.