$700 gives you some choices. I would get a dual exhaust. You don't have to worry about it being too loud as long as you are just installing a exhaust kit and nothing more(like remove the catalitic converter). Look on ebay, when it comes to exhaust I find it is cheapest to buy from Ebay vs. local shops. IMO Magnaflow makes one of the best kits, Check this out
eBay Express: MAGNAFLOW 99-04 FORD MUSTANG V6 Cat-Back Exhaust 15717 - Description. Pypes sells a cheaper kit but it is no where near as nice.