What is the power consumption (voltage & current) of the cameras?
By POE do you mean that power thru the ethernet connection (as opposed to controlled...)? I ask in case the CAT-x can't handle the power.
And I presume you control the power to the box, hence the power to the PC that in turn controls the devices.
You need to specify such things, and if you want the AC-DC co-hosed (I suggest not) rather than having a single DC input or 2 DC inputs (DC from mains & 12VDC) with (say) AC priority.