Back to the original question, though...
$300 to $400...so let's say $350.
Use that Kenwood amp and one or two of those subs. Subs done.
Don't worry too much about a high end head unit. On such a limited budget, you're better off putting the money into a decent amp and good speakers. Look for a unit that has a built in crossover. A unit with a built in crossover will have an output for your fronts and a seperate output for your sub, and, obviously, will take care of the crossover. I'm going to guess $50 - $75 here. My personal preference here would go to Alpine. I scanned some Alpines that closed on eBay recently and found a bunch that sold for just over $20, and I saw a 7940 (nice unit!) that sold for $42 including shipping (Although, admittedly, I forget if that unit had a crossover in it. I think it did). Let's go with an estimate of $60. Leaves us with $290.
Get a small, good amp. Just stick to a decent name brand like your Kenwood. Something like the older Coustic amps or the PPI Sedona series pop up now and then - they are examples of really nice amps that you could snag at a (probably) excellent price. I would think you could nail a good amp on eBay for $100. Leaves us with $190.
A random search for speakers quickly turns up a pair of
CDT Classics, new, for $100.
Which leaves you with a bit less than $90 (counting shipping for those CDT's) for wiring and other installation materials.