What you've got there looks good assuming you'll be buying that stuff from eBay. I can't seeing you being satisfied if you buy retail equipment at those prices.
Also, if money is tight, I'd skip the replacement of your rear speakers and just run the stock ones off your new head unit. I was surprised at the sound of the stock speakers although they don't look like much, they sound surprisingly good.
You're on the right track with a 4 channel amp, bridging two to a sub, and running the other two channels to your front doors. Just be sure to get coaxials up front so you really get the highs. Directional coaxials would be even better. ...for that matter, if money is no option, I'd probably spend the $100 I saved on not replacing the rear door speakers on better quality front door coaxials.
...and about the heater vents, you don't have to remove them. I'm sure if you spent some time you could work the amp under there keeping the vents in place. I just didn't care that much so I took them out. Besides, they can be reinstalled very easily if I change my mind, I didn't have to cut anything, the vents just pull right out.
For an amp, I'd recommend the Kenwood KAC6401. It puts 60W RMS to four channels, or 60W RMS X 2, plus something like 150W X 1 in tri-mode. Plus it's got a built in fan, and they're affordable.