Your head unit is nothing, if they charge you anything over 50 for that alone its a rip off, you can do that in a few hours or less, i could do it in 30 min, but i live in colorado. the subs you will have to get a box, unless you already have one. See if anyone is selling custom boxes on this site, or newcelica.org, thats what im looking for right now, the enclosure i got is way to big, i got the subs used for 75 from a friend out of his saab.
Amp(s) are really easy, however you might need a power inverter also. Unless you dont carry anything in your trunk you will have to place everything in good locations, like the power-inverter within the sidewall, close to the rear lights, and the amp, right on the box, so you will not damage it carrying a bike or some other junk