Wotcha and welcome - here is a buyers guide what to look out for.
http://uk.subaruownersclub.com/forums/topic/1619-help-check-guide-buying-1st-subaru/
some basic differences
wrx sti tends to be the older models
wrx quick turbo car dependant on year 5 speed gearbox
sti similar to wrx model but can come with six speed box, more power as standard, more powerful brakes, wider track and alloy components instead of steel used in wheel mounting gear
there will be other stuff as well but that gives you a brief high level