If you want to do a full system overall then that's when it gets expensive.
You can either do a 2-way component in the front and eliminate the 4" mid; a 3-way component in the front.
If you go passive you just need a 4 channel (1 channel for front left, 1 for front right, 1 for rear left, 1 for rear right).
If you are running active (first of all you'd need a processor), then you'd need a 6 channel if you're doing active 2-way in the front and coaxial in the rear. If you're going active 3-way front then you'll need a 6 channel for the front, and 2 channel for the rear.
Personally, if you're on a budget, I'd do passive 3-way up front (still gonna be somewhat expensive depending on what you get), coaxial in the rear, a nice 10" or 12" and a high quality 5 channel. Run a new headunit or do a simple 5 channel processor (such as the Audison Bit Ten) and keep the factory radio but get the amazing sound upgrade.
Feel free to hit me up if you want some product suggestions and pricing.