the Arr apps will automate downloads but you can go into their ui’s manually for overriding things when needed (like replacing a bad copy of a TV show for example), jellyseer/overseer handles requesting and adding new shows/movies to be monitored from a simple webapp that you would host on the server and give them a shortcut to on their devices homepage.
I’d go with a 12th gen or newer intel cpu, something small and entry level is more than enough like a 12100 or 12400, we just want the igpu to handle the occasional transcode, 16gb of ram, a cache SSD or two in a mirror, and a decent stack of HDDs of your choice, the OS can be anything you want but I suggest going with something NAS focused like unraid, openmediavault or truenas (jellyfin is not officially supported on truenas but it does work). if it’s a new build from scratch for long term archival of high quality media i’d start with at least 6 HDDs, with one for parity, if you can budget for 20tb drives for example that gives you a spacious 100tb of useable space with the ability for any one disk to fail without any data loss. you can then build that into a normal ATX PC case.
You can use windows or any flavour of linux but you will be doing more work to make them work properly, where the above solutions are more plug and play.
I would make sure their hardware is capable of playing as many file formats and codecs directly as possible though, when you get into hosting 4K media, particularly for full fat UHD Bluray rips, you will find apps built into TVs or lower end streaming boxes just cant do it and the server has to chug through transcoding on the fly, the igpu can do it just fine, but you should try to avoid it for maximum performance and image quality, so perhaps budget for an nvidia shield or something.