hey chun welcome.
for tips on purchasing a wrx, read through this:
How to Buy a Used WRX or STI
as far as simple engine mods- i think you'd be happy with a turboback exhaust, catless up-pipe & a tune. this will yield about 220-230ish WHP
for the exterior- that's up to your personal tastes. a nice set of wheels, springs, & a front lip is usually enough to hold me over. oh and a wingless trunk is always nice too