Best hardware for installing HA?

+1 for Ubuntu server.
I am running an Ubuntu server minimal install VM dedicated to hassio.
This is on ESXi hypervisor on the Xeon version of the cheapo dell T20 server.

@MarkR - actually you make be surprised how simple hassio is.
There is a good discussion of hassio on ubuntu here including input on a minimal ubuntu deployment:

If you have docker on ubuntu then you have huge flexibility to deploy other things that are not yet supported as hassio addons (plex, pihole, openVPN etc etc)