Firstly welcome to the site. You'll find most people around here are friendly and always willing to help.
Secondly, browse thru some previous threads, especially those in the audio section for system tips.
1. For your system, for ~3-500 you can get an entry level head unit with some half decent 6x9s and that about it. Look for a package deal at strathfield or jb. Also, you gotta allow for installation unless you're gonna do it yourself.
2. Dont rape your car and bolt a hideous wank style aluminium wing to the back. A kit would be hard to come by. Not sure if trx components fit on the corsair, but even if they did youll have a bugger of a time finding parts, then youd have to pay out of your ass for it. Theres a thread somewhere around about EOI for custom front bars, check that out. A nice neat spoiler though should be pretty easy to come by. grab a trx one from a wrecker. not sure of cost, but you may also need to paint it.
3.Seats are entirely a personal thing. Go and test sit some at autobarn or wherever and find some you like.
4. I would suggest any funds you allocate to neons be transfered directly to getting a better stereo. IMO neons look cheap and nasty, with some execptions though. But if you insist toddle down to autobarn and check out hat theyve got on offer.