UK advice for a small/tiny form factor

So, we've just bought a house and now have a proper garage. I am looking at putting a much bigger server cabinet in the garage with a fibre cable into the house to my office. I have a Dell Opitplex 3070 with an i5 9500 but I want to expand the home lab to something like 3x Lenovo Thinkcentre. 1) Low power consumption 2) To learn High Availability 3)To expand what I'm running. What are some cost friendly computers I could buy off ebay to match the hardware across all 3 for Proxmox HA? I am thinking 3x LENOVO THINKSTATION P320 TINY. Any suggestions be welcome and appreciated! :)