Use whatever you have and test different setups. I would start by installing Proxmox and setup ZFS on some drive that not the boot partition. For just checking it out with some lightweight VMs and containers any CPU that’s not 20 years old will suffice, the more RAM the better. Play with VMs, backups etc in small scale. You can use your old external USB HDD, etc, just to figure out things like ZFS.

Don’t buy anything before getting some experience and having some kind of plan.