Interesting design, thanks for sharing your thoughts! At the mill the main server is connected to a battery pack, so that if the mains power goes out the computer will run on the battery, and the battery pack sends a signal telling the computer to shut down in an orderly manner.

For the backup computer I'm still studying how to customize a iso image. I think it would be nice to have a minimal OS and all the backup scripts in a single iso image, so that if I ever need more copies of it I could just write the iso image on a bootable usb memory. So, the backup system itself would be safely backed up, easily replicated.