I'd get a regular old rex and make it stage 4 with an EJ257 motorswap(engine in the sti) if you so choose.
With an sti you get:
brembo brakes + 17' BBS rims and Potenza max perf. tires
STI 6 speed transmission
2.5 liter sti block vs. 2.0 liter stock wrx
Sti mounts etc.
sti exhaust w catless up pipe
VF series turbo
Red gauge cluster
Brightly colored sti seats
It would be cheapest to mod a stock wrx to better than sti standards but the sti comes with a warranty for all those extras. Also if you do choose to go with the STI look on Ebay they are selling them for as low as 27g's. Hope this helps and next time remember to post such questions in a different forum