I can’t say this is a good advice by itself; this is simply my setup as I was in your position just 1 month ago, maybe this gives you more ideas.
I recently bought an 16 GB non-ECC DDR5 (which is unnecessary, DDR4 works just fine) open-box 4-disk bay TerraMaster as my main server for the equivalent of around €400, thinking that in a distant future I can buy a different rack just for disks. This model has an Intel N150, which has the H.265 codec, which should handle the transcoding in your Plex instance (which, IMO, you should consider moving to Jellyfin instead in the new computer you end up getting). The 4-disk bay (there are some models with more bays) allows you to get some disks now and then fill up the bays later on if you want to. Note: I just realized this almost reads like a Terramaster ad, that’s not my goal; you can search for similar options from other fabricants.
As for the OS, I insist in recommending you TrueNAS, since it’s Debian-based, since it’s not like Proxmox where everything has to be VM, it’s simply Debian with a nice UI for spinning up Docker instances + disk/snapshot/backups management, all of which are optional: you can easily mount your disk pool and setup them up as a ZRAID (data redundancy in case of disk failure), stripe (no data redundancy), etc., organize everything in easy to use folders, schedule different snapshot schedules for different folders, etc. You can also easily mount Docker containers, either through the “app store” (a selection of Docker containers wrapped in a nice UI for configuration) or manually with docker-compose.yml files. IMO, you lose little, but gain a lot with the OS being already configured for a lot of the stuff you want to do, and the easy to use Web UI.









I got mine open box off from eBay. You could search in a similar way. I hunted for a few days, with terms like “NAS”, “N100”, “N150”, “MiniPC”, “4 Bay”, etc.
I’m not sure on this, so take it with a grain of salt, but I think non-NAS oriented mothebaords aren’t optimized for low energy consumption, specially on idle.
Best of luck!