Since you've mentioned you want a single computer to play media you already have, forget Plex. Sure it can be used in this manner but it's overkill. Plex, and other software like it, are best used for streaming from server to client(s).
Building on the Ras pi suggestion, I'd say take a look at osmc on an rpi. (If you go the rpi route, I'd also suggest dropping a lil more cash to have it boot/run from something other than an SD card, ext hd and nvme SSD are both better boot drive options. SD cards have a tendency to bite the dust after a while in single board computers)
Also, since you mentioned being unaware of the steps you should follow to get something like this running, here's a quick guide on how easy it is to get osmc running: https://learn.littlebirdelectronics.com.au/guides/create-a-htpc-media-centre-with-raspberry-pi