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Linux May Be the Best Way to Avoid the AI Nightmare
(www.lifewire.com)
This is a most excellent place for technology news and articles.
My plex server is headless, running Almalinux. Doesn't take much, I have it running on a very old NUC8 (NUC8i5BEK). The box is also running Asset UPnP and AudioBookshelf server too.
Personally, unless the server will also be the client (as in, you'll be watching from the server box and not a streaming box, tablet, TV app, etc), I'd skip any GUI and just install it from the terminal, save your resources for what matters. Desktop environment is pointless for a server machine.
If you were buying a cheap machine to handle it today, I'd probably recommend a Beelink (or other) mini-PC with a Ryzen 5000 series chipset (5500u/5560u models with 16GB RAM can be found very cheap, generally $215-$240 new these days). The 5000 series in particular are very power efficient for something you likely will leave on all the time, and have both 6c/12t and 8c/16t variants, though the 8 core ones will probably be more like $300-$320.
Whatever you buy, if it comes pre-installed with Windows, delete the OS. I wouldn't trust preinstalled on these boxes, and in any case Microsoft is getting really sketchy with this whole Windows Recall thing anyway.