Building my first nas server.

[TheDreadPirate]

This greatly depends on what you need. How many SATA ports you need, how many and what kind of PCIe slots, if you need more than 1Gb Ethernet. Just don't splurge for Z series Intel boards or X series AMD boards. They don't have any features that you'd need for a Jellyfin server except perhaps 2.5Gb Ethernet.

And the case also depends on what you need and the board form factor (ITX, mATX, ATX, etc). I generally recommend Fractal Design cases.