Run a VPN server on your system and expose that. Then connect to it with the VPN client on the remote system.

On the same network yes. If you want to do it from elsewhere, you should use a VPN, don’t expose stuff like Jellyfin to the Internet.

Take all of them and build a cluster.

Or whichever one is newest/has the best CPU, you didn’t really give the specs. And then take the drives and RAM from the others and max it out.

https://help.ubuntu.com/community/HowtoPartition/PartitioningBasics

You’ll need to provide details on what exactly you have for disks and how you want to use them if you want a recommendation.