Is that each for the subs? Anyways if it isn't just drop the speakers and do them later when you can afford.
Subs -
Kicker CVT102 10" Car Subwoofer - RYDA Car Audio($198.85) These because they are some of the few 10'' shallows that are 2 ohm, (or any dual 4 ohm dosen't matter) this will allow you to wire them down to 1ohm to make the most of an amp (providing the amp is stable at 1 ohm the one i have linked is of course)
Amp -
Fusion CA-DA12250 Car Amplifier - RYDA Car Audio($348.85) Watts per dollar 1100rms for $350 is good
Head unit - You choose, pretty much you get what you pay for, and most models in the same price range perform equally, it will come down to which one you like the look of the most, unless it has a specific feature.
Wiring - Gonna set you back about 100 bucks.
So $850 get's you the amp, subs and wiring, pick a head unit you like, get it all going, then later on look into upgrading your speakers, you'll probably want to listen to them on a demo board before you buy them, and decide how much you need to spend to be happy.