There are a variety of SBCs with a variety of ways to power it. Some power through a backplane, some ATX, and there even are those with a 12v plug. You're better off just determining how you would like to power one, and only search for those products.
As for ideal clock speed for carputing, that's not dependent on SBCs as this affects all carpcs. Your ideal clock speed is what you plan to do with it. Just mp3s (headless, even) you wouldn't need anything faster than a pentium 133, I'd imagine. You probably would want to instead ask 'what's the ideal clock speed for processing x, y, and z' as "carputing" covers so much ground and too vague to give any sort of answer for.