Getting a machine with lots of storage is first step. Make sure you also get an Intel graphics card. A380 I think. They're amazing at transcoding and color mapping (10bit->8bit). Transcode is very important to support with hardware, since it allows realtime resolution change, colorspace conversion, and pretty much every video file format to mp4. Can be done in CPU but not recommended.
I'd recommend Linux, but Jellyfin works with damn near everything. Use whatever youre comfortable with.
Getting content? First, use Firefox with Ublock Origin. Turn on all filters. Pirate websites are VERY hostile.
Get a VPN that can port forward. PIA and Proton can both do so, and are reasonable priced. VPN keeps simple copyright scanners from attacking your network, legally speaking.
Sites to look for goods: thepiratebay.org still exists. /r/piracy reddit has loads of links. Yandex search of "show torrent" has all completely relevant links. If you have a Usenet subscription to alt.binaries then you can use that method to grab stuff.
I also grab DVDs and Blurays and rip myself. Doing this is great since you may be able to find shows, but special features are almost never included. I use this tool to quickly rip: https://github.com/xenomachina/dvdrip/blob/master/dvdrip.py
https://www.bfloeser.de/posts/bluray/ covers Linux Bluray ripping, including where to download the AACS keys.
https://github.com/Jackett/Jackett | https://github.com/qbittorrent/search-plugins/wiki/How-to-co...
I abandoned MythTV and rented DVR from Comcast for just that reason.
This isn't as true as it used to be. My current Jellyfin server is an old laptop with a 5TB USB hard drive dangling off of it. Syncthing is used to sync the media to another system in my house because I don't really want to rip it all again if something fails.
If you proactively torrent "everything I may conceivably ever want to watch" or are glued to media 12 hours a day but still want it all to be new, then 5TB will go quickly, but 5TB can go quite a ways for people who don't match those conditions.
While a NAS won't hurt anything, the days when you needed one to do anything useful at all are gone. Just like I just slam my entire MP3 collection on to my phone's SD card now... I don't need any sort of windowing setup, or network access, or anything fancy anymore. Not only does it fit, nowadays it's only a fraction of the space anyhow.
You don't have to use this, but maybe it inspires you.
If you go the torrent route, there are also subreddits about trackers.
Pay for a seedbox and get access to a single private tracker site and that’s all you need.
I'm running it on the Windows Pro that came with the device. I'm not saying you should use Windows, I'm just pointing out that you don't need much to run it.
Huge list of self hosted stuff.
I personally run Plex(TV/Movie/music), Audiobookshelf(podcasts), Immich(Google photos), along with a bunch of other unrelated stuff like Home Assistant, Apache, etc.
It's all run off an beelink n100 and some NAS drives. Super cheap and useful.