I am trying to decide how to balance services, but running all 3 slightly less old computers will be less costly than the 2 very old computers I am currently running, due to them being on newer processor architecture.

I was definitely planning to have at least 2 running 24/7 though, specifically for having 2 pihole instances running, as well as having headscale and my tailscale exit node on separate devices (so that devices like my phone are still covered by the pihole).