What is being suggested are good choices and definitely the most powerful of
a system you can get. But I want to give a recommendation for EOMA68 if you
don't need a lot of power right now and can wait until next Spring:
https://www.crowdsupply.com/eoma68/micro-desktop
Whereas the old x86 systems that have been reverse-engineered are basically a
dead-end (they give you freedom now but without giving you a path to more
freedom-respecting systems later), EOMA68 is giving us a path for further
development, so I think it's definitely good to support it. Future upgrades
will also be cheap, which should make it a better long-term investment.