WIRING: I suggest getting 4 guage wiring,or 1/0 wiring and distribution blocks.
MONO SUB: I have the Infinity Reference 611a mono amp, and its birthsheet says it does 738W RMSx1 at 2 Ohms. I have had it for a couple of years and LOVE it! You can get one NIB for $194 shipped here:
http://cgi.ebay.com/INFINITY-611A-MONO-CAR-REFERENCE-AMP-675-WATT-BRAND-NEW_W0QQitemZ130224090130QQihZ003QQcategoryZ64570QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Highly recommend getting this.
SUBS: As far as subs go, I like having 2 subs better than having 1 sub, due to increased SPL and ability to upgrade the amp later (My Inifinity 611a does 738W RMS at 2 Ohms to push 2 subs rated at 800W RMS each at 2 Ohms- lots of room to upgrade the amp!).
FRONT SPEAKERS: To save money, you may have to get coaxial speakers. I have the Infinity 6012si speakers which are shallow-mount 6.5" speakers. You can find them on
ebay for $65, shipped. They are rated for 50W RMS nad have a mounting depth of 1-15/16", or 50 mm. A link:
http://cgi.ebay.com/INFINITY-REFERENCE-6012SI-6-5-SHALLOW-COAXIAL-SPEAKERS_W0QQitemZ350062205119QQihZ022QQcategoryZ18799QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
AMP FOR FRONT/REAR SPEAKERS: Get a 4 channel amp, rated for at least 50W RMS at 4 Ohms. There are tons of them out there at decent prices. Look around. Try Hifonics as a starting point.
HEAD UNIT: I suggest getting one with a front auxillary input so you can hook up any mp3 player to it. This isn't necessary, but greatly expands the functionality of your HU. Get the highest voltage output you can afford. Best Buy often has specials on their head units where if you buy one from them, they will install it for you for free on purchases over $100. HU's voltage output starts at 2 Volts.
Good luck!
EDIT: I am not a fanboi of Infinity; their products just happened to meet my requirements when shopping for audio components.
ENCLOSURE: Please, please, please make your own. It is a great way to spend a weekend afternoon and you will be SOOOO proud of yourself. All you need is some 3/4" MDF, wood glue, drill, 1.5"-2" wood screws and a jigsaw. Here is a GREAT site where you can design your own custom enclosure:
http://www.reaudio.com/speaker_box/LPort_Box_Calc.html
Once you have designed it, it will tell you the size of the cuts needed. You can go to Home Depot or Lowe's and have them make the cuts for you right there- usually for free. Then, all you need to do is drill holes for the screws, glue and assemble the box. Easy!