Honestly, the only parts that can’t be off the shelf are the nozzle, heat brake, heat sink, and some gears. So if you are halfway determined, buy whatever cheap-ass printer and swap their electronics to an Arduino mega with a stepper motor shield, like, that’s it, just plug and play your new control board in, flash whatever firmware and away you go. Hell, most of the super cheap ones are already just run on an Arduino clone of some sort.