Welcome to the club! Simple small and energy efficient: Beelink or Geekom. But those are the “new” option. My favorite upcycled servers are based on the HP Elite Desk SFF (Small Form Factor). My ProxMox cluster runs a few of these. You can look for the used ones with the lower TPU processor if you want them to suck less power.
Discussion
How do you manage the storage?
In my set up I use Synology. But I would recommend UnRaid. It depends on how much stuff you want to store. A few of the HP Elites have NVME storage expansions if you want to keep the storage in the ProxMox cluster. If you are starting small, I would suggest just starting with an UnRaid set up. Get use to it and learn about storage, virtualization, containers, etc. Then grow into ProxMox.
Thanks for the answer!