You got a great bike for a hell of a good price. Do your research on this board and figure out what mods fit into your budget in what order and go from there. There are some great suggestions in the Faq's section on this site. Ride safe and congrats on the purchase. Just keep in mind that once the mod bug bits it is hard to stop! ;D
best bang for the buck is sprocket adjustments. do a search and you'll find a LOT of info on that. everyone likes their pipes, so no matter what brand you choose you wont be sorry. i have termi's on mine, and i love em. always look on ebay for fun stuff too!
you can also do an open airbox mod i think, or get a K&N air filter.