Recommendations on Plafor for First Proxmox Build
Lenovo m900 tiny
They're small, cheapish, and low power.
This right here. The exact models I used when setting up a 3 node PVE cluster. Ram is expandable, multiple storage slots, good IO, and best of all they are CHEAP.
Whatever you have lying around. Keep everything in VMs and then it is easy to backup and migrate to new hardware at a later date. Good luck!