I suggest reading the FAQs to get a good basic understanding.
Also the P3 will be sufficient. Some of the popular Via C3 and C7 CPU's on lots of CarPC motherboards, are equivalent to P3 700Mhz (1Ghz C3) and up. So you will be ok, but not mind blazingly fast. So dont expect awesome 3D effects and visualizers, but for basic CarPC and GPS and Video Playback, it is what I used to begin with.
As for cheap: eBay