I would lean toward an 8th-gen i7 for 6 cores and 12 threads. Look for a off-lease Dell Precision 3430 on Ebay, they are essentially an i7 OptiPlex with a slightly larger power supply and an add-in low-end video card such as a Quadro P620. Not a lot of extra power consumption, but it offloads the display to a GPU and you may be able to farm out a few cameras to the Quadro if the CPU can't quite keep up.
I attached some screenshots from an i9-9900k with a GeForce GTX1660. Power consumption is substantial but electricity is cheap here. The first screenshot shows it recording direct-to-disk on 1200MP/sec with the console open. The second screenshot shows it replaying 12 of the cameras on our building at 1x speed, using the "Fast" (jagged-looking) display scaling, and as you can see, playback of these cams approximately doubles the CPU load. 2x playback speed is beyond the i9's capabilities here, although to be fair I run high framerates on the cams.
The third screenshot shows 1x playback of all cams at once, including the ones not shown in the prior screenshot, some of which use nVidia acceleration. Now you see the nVidia usage jump, while the CPU is maxed out.
So without being completely scientific, scaling down to your estimated 800MP/sec, I'd guess an i7 plus a low-end Quadro could work if you are reasonable about how many cameras you play back in a group, and use framerates that aren't too high.