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.
For my first server, after moving on from 2 raspberrys to a Proxmox host, I went with an embedded Asrock MB, passively cooled so you know it wasn’t drawing much power, still had multiple SATA ports and with the right sticks I could get 32GB RAM in.
Seems better to me than a minipc where you have no expandability, especially no chance for RAID.