As a starting point and as long as you understand that it's a playground that you can make more serious down the line, literally anything will do. Dabble here and there with the software, but I agree with the fact that having room for expansion (i.e. expansion cards) is a nice bonus. As a rule of thumb I'd say that at least four cores (hyperthreading would be good to have but not mandatory) and 16 GB of RAM is a decent starting point that will allow you to run a good bunch of containers and VMs in Proxmox.