From experience, the Sub(s) don't need to be in the cabin of the car, the vibration can be felt fine without then needing to fire in to the cabin, so leave them in the boot. Rear speakers are just a little bit of fill when you are driver and you can live without them. As I normally end up with the fader slightly to front to balance (to my ears) the sound from front and read, the rears don't need to be great, you may get away without upgrading if you fade forwards as they won't be getting the full volume hit.
Spend your money on the front speakers, that is what you'll hear. You don't need massive woofers for the front, in fact a decent dedicated 4 inch mid and a separate tweeter (with a proper x-over, not a capacitor) will sound ace, as it's not even attempting the bass. Obviously this will sound a bit hollow and empty without a sub.
If I can help at all, just ask.