If you plan to taking any images of small detail ( like part
numbers on a carburetor ), I'd suggest bringing a US dime
with you to the camera shop and taking a few images of it
for compartive level of detail. Many digital cameras tend
to shake and blur on max zoom settings so you should use
some do-diligence here. Most any digital camera can handle
standard car shots with ease. When you start talking about
mirco detail, well let's just say that this separates the
men from the boys.
As others have already suggested, anything over 5 m-pix will
probably suffice for your needs. Remember, the higher the
resolution, the bigger the images ( file size ).
Let us know what you end up with. I'll be in the market
soon myself........
Steve