I run Plex on a Raspberry Pi 3, it can support two simultaneous 1080p Streams on my local Wifi. Cant support 2k or 4k videos at all. And cant support video outside of the local network.
“use your favorite Unix then install Plex” or “Here are 56 perfect versions of Unix to install for your Plex server”
What part of this do you think is hard?
Each step can be scary at first but its not hard if you break it into pieces.
Booting Ubuntu or some linux OS is a fun first step if you actually have a spare computer handy