You aren't going to get much for 400. First thing I would do is get a cheap head unit (easier and sounds better when hooking up an amplifier) then run a 4 or 8 gauge amp kit depending on how many watts you want. If you can do this yourself you're at about 150 or so. That leaves 250 for amp and sub/subs. You can do it. Just might have to buy used and somewhat lower your expectations. If you have to get all this stuff installed you better save up a lot more than 400 bucks.
He can do a line out converter on his stock head unit on the cheapz, hell he can get a LOC at wally world for $15. I'd place more money on just one 12" sub and a well made enclosure, and a half ass amp. I'd suggest a Hifonics amp, in testing we've found they're no less worse than more expensive amps in the same catagory.
But definately make sure your enclosure is built well....you could buy the best amp in the world and still have your setup sound like shit because of the enclosure being not designed well.