DIYMA.com is a great place to start reading daily. if your willing to do the research you can build a system for $200 that would sound better then most peoples $1000 brand name systems. there's a great classifieds section..always tons of great equipment for sale. hell, if your desperate there is a "for free" section also just pay for shipping. It can be done.
If you want all new stuff for some reason you could do something like,
two mids spk - $16 Peerless India W6-KA638-SH 6.5" Paper Cone Shielded Woofer | Parts-Express.com
two tweeters - $14 Dayton ND20FB-4 Rear-Mount 3/4" Neodymium Dome Tweeter | Parts-Express.com
5 channel amp- $139 NEW CRUNCH P1-5000.5 1500W 5 Channel Car Amplifier Amp - eBay (item 390257607589 end time Nov-28-10 1455 PST)
Any cheap sub will do PE had a $14 big name sub that would have been perfect. it was a limited time special (which they always have, especially around christmas time) that only leaves 16 bucks for a box and wire, but if you can catch that amp on sale then you should have plenty of money for that stuff. monoprice.com would be great for wire.
Thats about as good as I can do, someone else might be able to save some money somewhere but its just something to think about if your interested.
for the headunit you could just tap the wires of the factory for a balanced high voltage signal. or just plug an ipod into the amps rcas.
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope