Don't buy anything for the rear and get a 6.5" set for the front and the front only.
When you have more money to spend, get an amp for the front speakers.
My Rec's are in this thread.
http://www.toyotanation.com/forum/t178548.html
For a headunit, I'd only get the Pioneer Premier DEH-P780MP and the DEH-P880PRS. I'd get the PRS if I wanted to wire active and the 780MP if I didn't plan on it(get the 780MP)
For a sub, heres some ~100 dollar subs I would rec.
TC Sounds db-500
http://www.tcsounds.com/db500.htm Only 75 bucks for a 12" and it's got a TC2 motor.
If you got a little more, this sub is VERY good.
http://www.tcsounds.com/tc9.htm
The phoenix Gold RSD sub gets very good reviews and it's pretty cheap (~100 bucks)
The A/D/S Reference sub is a VERY good SQ sub for cheap.