Is there any proof that such potential backdoors are practical and not theoretical? Have you considered non x86 hardware. If you were made of money for example Talos II or other more reasonable priced options?
For a given task, newer computers are more efficient than old computers. My 20 watt Intel NUC easily fills a role that I used to fill with a 200 watt surplus PC.
Whether or not an upgrade for efficiency is worth it is subjective though.
