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.
A better value is just getting 6th gen or newer Intel CPU and using its built-in GPU to do the transcoding. If you want a discrete GPU, any low powered card that supports HEVC should work. Alternatively, you can get something like a Roku device to connect to your TV as they have pretty good compatibility and you’ll avoid transcoding all together.