Western Digital Reds or Seagate Ironwolf iirc. Yeah they're expensive because they're rated for a different use case than traditional drives. I'm no expert though! I believe I've got a WD Red.
I believe thats what I do though, I have a ssd in there as well for the base os. And I just have the nas drive mounted. Jellyfin docker container setup to point to the nas drive as the media storage folder, as well as a samba share setup so I can throw stuff like bandcamp purchases easily.
This is all off the top of my head, as I'm not at my server atm...
I believe the 12400 should be good for hardware transcoding. I have a dell optiplex (i5 9th gen, can't remember exacts) running a low traffic server (my wife and I), with no issues running at least 2 4k streams concurrently. It comes down to what you encode your videos to.
Additionally I'd grab at least one new NAS drive, as they tend to be rated for long term use. If options aren't great there, then maybe a few drives in a raid configuration may also work. If you do get a NAS drive make sure its fairly quiet, my server is located in a common room, and the hdd fan is fairly audible. oops.
oof. Yeah I think I have the 5400 rpm one. Maybe start with the regular hdd in raid and see how that fairs long term. Just make sure to backup your media in more than one place!