Ford Focus SE 2014 Budget Build

I just got a brand new 2014 Ford Focus and looking to upgrade the audio system. Probably going ot keep the stick head-unit as Sync is integrated. I'd like to keep the budget below $500.

Front/Rear Speakers
6.5" component w/tweeter in front
6.5" speakers in the rear I think the door speakers can handle 6 3/4" speakers too?

Subwoofer
I'm not sure if I will have enough volume for a 12" subwoofer? Probably going to fiberglass my own box (Never done it before), and put it where the factory sub would be located if I had one. So shallow mount is a must.

Amplifier
5-channel amp as space is at a premium. Been scouring eBay/Craigslist tp find some good deals. Had luck finding an old Alpine T-class amp for a boombox I built a while back, maybe I will again?

Thanks!