I'm not really sure how much deadening would cost. But, for the trunk lid, I'd guess 2k+. And it would take only a few hours at most.

You CAN put 2 12" Targas ported if you go below the recommended box specs. But you will be losing potential of your subs. And you might get a different sound that you may not like. According to other "Targa people" 2.5cubic feet is the right size for a single ported Targa. Meaning..if you go 2 Targas..you're going to lose your trunk space.

Spare tire? I don't have one hehehe..it's unusable, so, I removed it. But, for big box users like myself, you have to remove the sub box, and lift up your trunk floor to get to the spare. But,if you think about it, you only use the spare tire a few times a year.

I'm using a 12" Targa DVC4. I'm over powering it actually hehe. I'm feeding it about 300-400 watts. But, if you feel the need to go for more power than 1 single Targa, then you can go for a 400-500watt single sub instead. But, sir, if you haven't heard a single Targa sub yet, I think you'd be surprised with its power, if powered correctly. When I'm at the mall parking lot, and I turn up the music, I can set off alarms with my single sub. Gumagapang siya talaga. I was driving to my titas house, and she said she could hear me coming from down the street.
Here's a vid I made a while testing out a new video camera...the camera can't pick up the lows, so it sounds distorted. But it isn't distorted like that when you actually hear it. This was before I setup my trunk, so it's kinda messy with the wirings
http://youtube.com/watch?v=7W6mriLyxlQ