Lastly I don’t publish too often… I’m so busy.
To modify common x86 hardware to server, I do this thing:
Power: the server input is in general 2 (I can do more but 2 remain simple), I chose 220VAC to be compatible, but I prefer 48VDC (22VDC to 52VDC). Why DC? I can add power source with simple diode, the step down DC converter is cheap and efficient, UPS is plugged in parallel (not in series like traditional 220V) mean can be parallelized and hot plugged without shutdown.
Why this range? I can use car batteries (2x 12V in series to have 24V), solar panel without battery bank, 220VAC to 48VDC power supply, final power bus can indicate to any device the remaining battery via the voltage, and you can manage priority with the voltage, 48VDC to the primary source, 40V for less priority, 24V for the lowest priority.
Watchdog? Yes, you can do it with ESP32 + relay module, just connect to USB header and ATX reset header on motherboard and load the software. I publish it into git of confiared.
Keyboard/mouse/display? You can do a converter to network o just buy it. I prefer NOT use directly the hardware and add LXC/qemu layer, then I don’t need more hardware. And you can setup remote control directly on host (VNC, ssh, …)
For high availability, prefer have 2 server into 2 data center into 2 providers. RAID software are always welcome, but FS checksum like btrfs can detect offline corruption (when you change bits offline or via liveCD)