I'd probably track down some sort of door pods - they would require the least amount of cutting, bending and installing. Certainly someone makes something to get some 5 1/4 or 6 1/2 speakers up front, probably even separates for better sound.

A single 10" sub would probably suit your needs - or even a very nice set of 6x9's *could* do it with a good amp. If you are starting from scratch, a 12" is the best all around sub for a single - in case you want to go louder.

I'm running a 20x4 and a 30x2 in a jeep wrangler and the system is plenty loud. The only issue I have is that my 8" sub can't handle too much bass at too high a volume. 40 or 50 watts should *easily* be plenty - in my opinion. Having too much power isn't a problem, it helps keep the system clean at higher volumes, but it does increase the mounting size of the amp and the costs. Listen to some setups in the store to get an idea of the relative volumes.