I just finished moving my Channels DVR installation from a Windows 10 PC to a QNAP TS-251D NAS ($229 on Amazon), and find it entirely satisfactory as a self-contained Channels "appliance". Two tips to pass along:
QNAP seems to think you are some sort of Data Center, and pushes toward a Thin or Thick Volume setup in their defaults, which is needlessly complex (and lower in performance) -- just set up a Simple Volume.
Transfers are much faster if you use a 2.5" HD in a USB 3 Adapter as the source: the "Upload" function from a direct network connection to the old machine only manages 20-25 MB/s transfer rate, which takes an enormous time for the 1.2 TB I had to transfer. A USB drive plugged in to the TS-251D manages 125 MB/s - 5x better. I accomplished this by copying my old 3.5" RAID array to a 2.5" HD plugged into the Windows machine. This copied overnight. I was then able to move the drives to the TS-251D and re-use them, transferring from the 2.5" intermediate in a few hours -- less time than it takes the new RAID set to sync, which happens seamlessly in background.
The end result is a one box "appliance" that performs well and handles 4K, at a low price.