I thought this video was rather interesting, because at 12:27, the presenter crunches the numbers to find out how many years it would take for a new computer purchase to be more environmentally friendly (in regards to total CO2 expended) compared to using a less efficient used model.
Depending on the specific use case, it could take as little as 3 years to breakeven in terms of CO2 if both systems were at max power draw forever, and as long as 30 if the systems are mostly at idle.
Assuming it’s compatible with the hardware at all, it should. You would have to install it on the Proxmox host itself, but Proxmox is basically just Fancy-Debian-for-Virtual-Machine-Hosting and it has Debian packages so that shouldn’t be a problem. Login to the Proxmox itself and install it there.