Another thing to keep in mind when shooting interior photos on a vehicle with your own digital camera is that you will also have the opportunity to do some post processing of the photos.
We have used both Aperture and Lightroom in our photo editing (as well as my own personal photo touch ups) to adjust colors, white balance, etc. Sometimes what a photo needs is a more diffuse light source and then to clean up the image quickly in post production. Don't know what your budget and time allowance for the photos are, but it might be something worthwhile to look into.
As a couple other posters have mentioned, make sure that your camera isn't just a digital zoom, but it should have something of an optical zoom as well. 4 or 5 MP will be more than sufficient, seeing as how these are just going to be used on the web.