Basically just the CPU fan, case, and PSU (if you're lucky). If someone can figure out how to get Doom running on just those 3 things, young people will be able to ascend.
Peripherals are dirt cheap now. I remember when I built my first pc and watercooling would cost as much as the CPU and the case as much as the psu (and that was just a basic metal box, before windows became standard)