A Virtualization Host server provides the hardware for virtual guest machines. In this scenario, I recommend focusing on CPU and memory used in the system. I also typically store my VMs on a NAS server using NFS which Proxmox supports natively, so I'll add a separate NIC card that is used for communicating with the NAS server for disk related traffic.
If you are going to store VM guest locally:
If you are going to store VM guest on a NAS Server: