I was lucky I had a couple of old ones here and some spare memory chips.
If you are going to use it, I’d just install PVE, then run emoncms in an LXC container. Some one pointed me to this guy recently - Proxmox VE Helper-Scripts | Scripts for Streamlining Your Homelab with Proxmox VE for easy to use scripts - install Ubuntu then use the OEM emonscript to install emoncms. Running PVE will draw much less power than Windows (even if in a container or VM). Spinning Discs draw a surprising amount of power as well.