You could probably do this a lot easier and cheaper with a dedicated standalone dvr recorder. You didn't really mention you wanted the pc for anything else, though I'm assuming you do. If you want it to constantly record multiple high res sources its going to have to be a competent pc, which means heat, size, etc. as you know.
I've got a single channel dvr recorder that automatically erases old footage and records constantly onto a sd card. The thing is so small you could fit it easily under your dash and plug any vga camera into it. You could do two of these and have the whole project done for under $200-300 if recording is all you want. And it will probably be a bit more reliable tbh, no crashes, hardware issues, moving parts, etc.