Sonarr with nzb360 to manage it. Completely hands-free, couldn't be easier to use once it's set up.
If you don't have your own hosting, search for "seedbox" hosting - there are plenty of great, relatively cheap options. I picked a hosting option which had the above apps already set up and ready to go, which made the whole thing seamless.
Instead of using a server, you can run Sonarr on your Windows desktop and have it provide the content directly to a local Plex.