The minimum spec is whatever e-waste you can find that still powers on.

My home server has an i3-4160, 10 gigabytes of mis-matched RAM, a ten-year-old 240 GB SSD with 36000 hours on it, and three 1 TB hard drives in a RAID5 array each with ~25000 power-on hours. It runs Proxmox on the metal with a virtualized OPNsense, Nextcloud, and Jellyfin server (plus smaller services). Jank levels are high, but not fatal, and it was mostly free.