or you could just buy an ipod and a good set of headphones?
Troy has the right idea with the sound deadener, it makes a huge difference especially with no sub.
tbh get a decent set of 6.5s for the front, then 6x9 at the back. shouldn't be too hard to fit them in, i've seen plenty of GTR with them (including mine? ).
you might try see if you can get a package: headunit, 4 speakers, the cables, and misc other crap. im sure your mate could wrangle that for 650
actually, http://www.jbhifionline.com.au/car-sound/car-speakers/pioneer-tsd1720c-17cm-packaged-component-speaker-system-260w-max/435849 + the 1st headunit is about $500, and really unless you wanted LOTS of noise coming from the back, that might be the best option. you are quite limited in terms of power from the head unit anyway, so im not sure of the benefit of running more than 2 speakers and 2 tweeters.