
Personally, I’d use the Pi 5 for your Plexbox and HA server. I have had success running both those on the PI 4 with boot from an SSD stick. If you’re unsure what OS, just use the Raspberry official Debian server image. Whatever you do, don’t use an SD card for your hard drive. It’s funny when people complain that the Pi sucks and they only use the SD card.
It’s like the Comic Sans version of The Ring.