To stay very economical... go with coaxial speakers in the front and rear, and power both off the deck. Get a mono amp or 2 channel and bridge it to a single 10 or 12" sub. Going with good quality products, that setup would run you around $500.
Stepping it up a notch, you could stay with coaxial 6x9s in the rear, and get a set of decent components up front. Using a 4 channel amp, power the FRONT speakers with 2 channels, and bridge the other 2 for a single sub again. Power the rear speakers off the deck. Doing this, you'll spend about another $200.
Another option would be the same setup as above, but get a 5 channel amp... power all the main speakers with the 4 channels, and use the sub channel for a sub. Some good 5 channel amps will run you about the price of a 4 channel amp... maybe $50-$100 more.
I wouldnt bother with Q forms in your case... just place the speakers in the stock location. If you get components up front, flush mount the tweeter in the upper portion of the door panel, just in front of the door handle.