I know you are on a budget but listen to what these guys are saying about saving your money man and spending it on brands that are true rated RMS Amps with no questions and supply power you are looking for. The 5 channel amp isn't a bad idea at all can be cheaper if you don't need a ton of power.
Honestly if you are looking for a cheap but good system right now. Keep the 5 channel amp. Spend a little more money and go with this kenwood. It will supply much more power for the sub.
http://www.sonicelec...KAC-7005PS.html It will provide true RMS power and be able to power one maybe two subs depending on your subs you go with.
Headunit go with a Kenwood excelon, Pionier or Alpine. Sometime buying a used one isn't bad. Be cheaper and you can find some good deals on craigslist or here. Even check ebay and Amazon.
Speakers: Go with Rockford, Kenwood, Infinity, Jl Audio. They may cost a little more but you will be more happy.
Remember the sub box. A ported box will sound louder if designed right. So just know what you want in your system. If money is a problem, which it is for most of us. Save money, look around and do your research and find good deals on the internet and stores!!
Good luck!