There are some great Linux packages that work very well, for CCTV systems. I don't know much more about this though, but you could ask at somewhere like ubuntuforums.org.
Ubuntu Server (or more ideally Debian perhaps, if you're more familiar with Linux) will be able to manage all of what you require very well, with good power usage.
As for a low-power computer, I can recommend the Mac mini. It works with Linux perfectly well, and consumes little power.
If you don't need that much processing capability (perhaps not, but I'm not sure exactly what you're doing with the CCTV data), a little machine with an Atom processor (running headless mostly) would be ideal.