4 speakers + sub is more of an SPLish setup. I wouldn't bother with the rear speakers unless there were passengers who demand them. Instead of buying and powering rear speakers, invest money into an active capable head unit, like Pioneer DEH-80PRS. The head unit gives you features of a DSP that costs twice as much. The cost break up would be:
Head unit:
Pioneer DEH-80PRS: $350 shipped with tax should be doable
Amp:
Alpine MRX-F65, powering front speakers in active mode ($300?) or PPI Phantom P900.4. I feel the MRX amp is a bit higher grade amplifier than Phantom, even if it does not have a super high watt output. Skip Infinity Kappa amplifer as they seem to have frequent quality issues.
Speakers: a set of Morel Maximo 6 or JBL MS-62C (both have very good reviews) running active. $200 for a set. I hear it's possible sometimes to get a good deal on speakers like Hybrid Audio Imagine from a local dealer, so that would be worth checking out (someone mentioned picking a set for $200.. normal online price: $300).
Storage: About $20-30 for a 32GB SD card drive for the head unit.
This will leave some $120 to spend on wiring, tools, sound proofing, and other projects.
If you run the front tweeters of the head unit amplifier (totally doable), you can power a subwoofer in future by bridging two MRX channels. That will give you just over 300watt for subwoofer. That's totally adequate to fill the cabin with bass, but not enough for SPLish kind of content specially if sub is in a sealed box.
Alternatively, if you don't need to budget for the SD card, sound deadener, etc. you can blow all ~$350 -450 left after buying amp and the head unit on speakers. I suggest Hybrid Audio Imagine, JL Audio C5, Focal Polyglass PS165, or Polyglass 165 V30, or the new 165 V15 model, whichever is in the price range at the moment.