If you’re interested in setting up another program, you can use Overseerr to feed requests to Sonarr. Other people who have access your plex library can also use it to request stuff from you, but even if you’re a solo user, it’s a nice interface to search and add content with.

ixSystems sells pre-built machines running TrueNAS. They’re a little pricier than building on from scratch of course, but they have ECC ram and have everything set up out of the box. Funds also support the development of TrueNAS. I got one earlier this year and I love it. Fussing around in the web UI requires some technical know-how, but if you get it set up for them, I expect it to run like a dream.

Other posters have described what Radar and Sonarr do, I just want to say having all the apps set up along with Overseerr is a game-changer, even if your setup is only for your own consumption and you’re not sharing your plex library with anyone. Overseerr lets you log in with Plex and request content, and it’ll add the content to *arr, which will automatically search torrent sites (I use Prowlarr for that), download the content, then move them to your media library and update Plex.

If you do share your plex library with friends, and can put Overseerr somewhere they can access it, then your friends can request to add content to your library, and you just have to click “Approve” to start the search & download process.

It takes a little time to set up, but once it’s up and running, it’s lovely.