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.
I’ve had great luck using Intel NUCs as home servers and HTPC boxes. Since those are now gone, I have found that Beelink is the most cost effective replacement when I needed to revamp the setup. My biggest complaint on them is that the cooling fans on them are not super reliable and it is not easy to find compatible replacements. I had to order them direct from China and there are a few wiring incompatible variants. I ended up with one of them being the wrong type and I had to resolder the leads to match the existing broken fan.