An early 2000's Ford Crown vic. With the right engine mods (new air intake, exhaust, aluminum drivershaft, and a good rear-end gear ratio) they will get above 20 MPG as long as you take it easy. My dad's 04 gets like 24 or so with those same mods. The aluminum driveshaft and intake (from a caddy with the same engine as his) cost him just over 100 and the exhaust was around 100 because we had a good friend do it for us.
The KBB value on an 02 Ford Crown vic is between 5,000 and 6,000. They're trunks could easily fit 4 12" as long as you know what you are doing. I believe there's something like 12 cubic feet available in the trunk without the spare tire.