I was in the same boat you are in an replied to a similar question with this answer. In my car, this setup sounds amazing. Although you can hear it from the outside, you would underestimate how much bass there actually is inside the car:
I just installed my first system for a low price. I used:
2 12 inch Pioneer TS-W301R Subs (~$60 a piece, walmart, 150W RMS)
1 Sony XM2002GTR 1200 Watt amp (200W RMSx2@4ohm, $150, onlinecarstereo)
1 Pioneer DEH-1900MP headunit (22W RMSx4, $100, Walmart)
I custom built the box to match the speakers. It didn't take too long and the color scheme matches the speakers.
After other ancillary parts, I ended up spending around $400. The bass is really good. I can feel all the hairs on my arms vibrating from it, pretty nifty.
And you don't need a $1K+ system for it to sound good. I wouldnt spend over ~$800 unless you are really concerned about SPL for competition. My subs most likely dont hit as hard as L7s or W7s, but it sounds great to me. Honestly, if I had any more bass, I'd be worried about rattling my car apart.
I wouldn't buy Dynamat unless you really think you need it. I had a few spots on the outside of my car that vibrated pretty bad, but some weatherstipping tape and foam pads fixed the issue.
The next thing I need to do is upgrade my door speakers from OEM. The bass is so overpowering that it's sometimes hard to hear the music...If I put any more juice through the stock speakers they pop really bad.