+1 for ThinkCentres, theyre basically perfect for this. quiet, cheap, sips power

if youre looking for something to make the software side painless too, I built Syncloud (syncloud.org) - yeah im biased haha. but its open source, basically an app store where you click install for Nextcloud/Jellyfin/etc and it sets up https and login stuff for you. sbould run great on a ThinkCentre