Totally get that issue. It might be worth spending a day or weekend making a boot-disk for them that will wipe their current disk image, and install win10 pro with it set-up so you can remote admin it.

That way for your parents they just have to buy the pc, put the usb in and reboot it, and then call you when it's back on so you can remote in and give it a key and finish configuring it for them. I suppose even that might cause issues if you can't teach them or walk them through via facetime how to set the device to boot from the usb instead of the harddrive.