While I absolutely support the idea of Proxmox (especially with ZFS), I think to start a learning project it is probably overkill to spend $600. I would maybe start with a Dell T30 or Fujitsu Celsius W550 for < $100 used or using a Gigabyte MC12-LE0 Mainboard with a Ryzen Pro 5600. Stacking one of these with a modern NVMe and some ECC RAM should go for additional $200, the Fujitsu Machine is OLD, but has intel AMT Remote Management and draws <10W Idle.