Ok, im in the same situation as you. I have a total of 1000 bucks in my system so ill tell you what i think.
Sub- $250 cerwin vega stroker 12' or 2 alpine type r's off
ebay for $220
Head unit- Definitly a pioneer or sony because the are only $150 and they are easy to use. I payed 130 bucks for m pioneer and wouldnt trade it for anything.
Wiring- Get 4 gauge wire on ground cable and hot wire. Get a fuse that is 80 amp or so.
I dont know about the sub enclosure, just make sure its ported. probably gonna cost about 200 bucks or so.
Amp- I would get a cheap amp rated at 2000 watts off
ebay or something until you can spend about 700 dollars on a good one.
If you get a cheap amp to start with you can afford everything else and still have enough power to push the subs. Also, install a couple tweets in the front to add to the highs. It makes the system sound better if you dont have really expensive speakers.
HOPE this helps.
CERWIN VEGA IS THE BEST BANG FOR YOUR BUCK. I SWEAR .