My advice to you:
1) Buy used. As you mentioned, check the classifieds, also try out zilvia.net
2) Try Craigslist.org as well - and ebay, but you probably already know that.
3) 600 bucks will buy you a good set of used wheels. Might I recommend a little drifter style? Maybe a different set for the fronts than the rear?
http://www.jdmenginedepot.com is the best place for imported stuff in NJ. You might want to try cali based places for better shipping prices...
4) IMHO: For 600 bucks, I would do the following:
a) Invest 100 on something. Anything. Try an orange savings account.
b) Take 200 and chance it at gamb|ing (hey, I live close to Atlantic City...give me a break).
c) Spend the rest on something nice for your girl. Then get the chick to pay you back physically (what...like you didn't expect that coming) and monetarily with gifts thru the year. Hehe...