Anything from 2004(?) to 2020(?) is going to have an Intel processor. I’d do some research to find the last user-serviceable year, then get the best processor I could afford. Upgrade the RAM and internal drive to the biggest your Linux distro can handle.
While you’re at it, check out the forums at your distro’s website. I’d bet good money that someone has already worked out what you need and shared that info with the community. :)