well, i have a quality exhaust for sale in the bst. its the stealth from focus sport. but to get a quality exhaust system, your gonna be spending anywhere from 150-500. 150 being a custom exhaust at your local shop. your headunit depending on what you want could be in any price range, just keep in mind, if you get a double din unit, the dash console will need to be modified. and the subs, well if your just looking for a little bumpage, go check your local pawn shop, just make sure there not blown or anything.
exhaust link: http://www.focusfanatics.com/forum/showthread.php?t=298901
double din HOW TO: http://www.focusfanatics.com/forum/showthread.php?t=231948