Building it up on a budget can be done provided you already have a quality, modern head unit. The key is to determine how many watts (RMS) you want to pay for and then stick with that volume of RMS wattage for the amps you intend to attach to the speakers.

I doubt you will benefit from the speakers placed behind the back seats so I would forget about that. This leaves two front, two rear and two subs. All that is left is a wire kit and a few speaker boxes.

A major retailer is selling a set of Insignia 12" subs with a two-channel bridgeable amp for just $199.99 USD. The setup will deliver about 125W - 150W to each sub RMS. Mine sounds great in a 78 regular cab F150.

With the subs in place, go for your second amp and front speakers next. Be sure to get a four channel amp so you can add your rear full-range speakers later. If funds are short, you could just install quality front speaker equal to the wattage for the amp you intend to buy. This will get you thumping for awhile until you get more cash flow to invest.

Using locan and online prices, I found...

$200 Subs and amp
$ 125 front speakers
$ 110 Wiring kit
$80 - 100 sub boxes
$150 4-channel amp at about 55W RMS per channel (This would mean you need around 50W RMS speakers for best performance). Remember, just match your wattages for your amps, your speakers and your wiring kit.
That is $685.00 and a weekend project.

$125 more for the rear speakers and you are done.
Total cost going down the road: $810.00