Only this morning I was thinking of how to make a cheap (really cheap!) bullet camera recording solution, and this maybe the answer, one of these, a cheap usb webcam, and hours and hours of learning linux
and it might be do-able
you might find there is already a software solution out there in linux. CCTV software packages are available in both linux and windows.
The problem you will find with such small computers is having the graphics processing to record high quality video and audio. You might do it with just one camera on such a system, but you might struggle to get high quality video off 3 or 4 cameras.
Im in the process of building a carputer to record upto 6 cameras and audio and i chose a micro ATX form factor motherboard so i can use decent processor and memory. Also it supports a proper CCTV card to give the graphics capability i want.