I’m genuinely envious of your setup mate!

I wish I had this many USFF PCs, looking around for Dell Micros, HP G series mini, Lenovo Tiny… The prices are absurd in the EU: stuff that’s 10 years old being sold for 150+ EUR

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.