I don't see these sort of computers as good value proposition for an always on server. These are decent machines, but an always on machine serving ontent has other things that need consideration.
if you're looking to build a plex server, I would massively recommend not using an Apple comptuer (unless you've got an old one laying around). Heck, I wouldn't even recommend windows.
A custom built "server" to spec is the best bet. I'm more than happy to help give advice on building out a purpose built server (as I do it for a living)
Right now I have my Windows machine as the Plex server. I have my Synology auto fetching based on RSS feeds, and everyday when I boot up my Windows machine, it will process the downloaded files. Ideally I'd love to not have to power my PC to do this since I turn it off nightly.
Ideally I'd love to get a Raspberry Pi sized Plex / media server.