They didn’t run it on a different machine, they ran it with the old GPU is installed instead of the new one.

You should not have to buy a new key for a machine that came with windows. That will most likely automatically activate back to whichever edition you had before the wipe.

A Surface might do, unless you find that to be too big for your liking.

Systemd basically is what starts up other processes. You don’t need systemd, but you do need some init manager. Kernel loads, then calls up the init manager to load everything else. Anything you want to make run on startup gets added to the init manager.