openmediavault and casas are good noob-friendly OSes for NAS purpose. Much faster and simpler to get it running than some Proxmox and True NAS overkill solution.

You can also just install whatever Linux OS you like, and plug-in some screen, keyboard and mouse and do your setup this way, like any other computer!

For Hardware, I recommend to build a computer out of standard parts. For what you said, a small motherboard with an integrated Intel N100 CPU and a nice looking case like the Jonsbo N2 will sever you well. This is very close to my current setup, using an older j5040 CPU and it runs everything just fine with no effort (Jellyfin w/ light transcoding, *arr stack, Usenet and torrent clients, syncthing, SMB and NFS filesharing, and more)