As far as I'm aware, you can't buy a prebuilt computer that ticks all of those boxes. System76 has Coreboot-supported laptops, but they don't offer it with their desktops because it's nearly impossible. And you can just forget about Libreboot, since it barely runs on anything. Neither is exactly ideal for modern systems, though...
The best you can do is build your own, but even then, it still requires the Intel FSP. Installing your own distro would be as simple as plugging in a flash drive with the installer when you're done.