This could spiral out of control, and I don't want you to waste money. Considering what limited knowledge we have about your listening preferences, start with the basics and build from there. Definitely get a new head unit and door speakers. If it sounds good to you, there is no need for an amp to the doors for now. A mono amp for the sub is the way to go. You need to factor in some costs for wiring, mounting, and labor if you're not doing it yourself. It's not that hard. This site has some great info. Good luck.
http://bcae1.com
http://bcae1.com