for light mod and most bang for yr buck
air filter: K&N air filter in stock box
Exhaust : personal i don't care much about brand, but has to be turbo back (split dump, high flow cat) X force is good and cheap
Intercooler - S15 stock cooler will be good for 180rwkw
Turbo smart dual stage boost controller
(or EBC if u got the $, i love blitz as the hold the boost nicely, hell lot better than the Greddy e-01 i used to have)
Spark Plugs - won't do much, but i use top of the range NGK
maybe some computer? SAFC, E-manage, uni chip even power Fc?
get a RB AFM as well once u got yr computer
after that, u will need fuel pump, then mabe a S15 turbo + injector