HA is an absurdly heavyweight pile of Python and Docker. Get it a real computer — a used “thin client” with 8 GB of RAM is probably less expensive than an RPi4 plus case and power supply.
What’s the issue with new ones? Proxmox should work just fine on new hardware.
I’m rocking a Dell Wyse “thin client” that cost rather less than any comparable N150. It’s fanless, still gets firmware updates (via LVFS!), and takes about 5W at the wall plug.