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.
Noctua coolers are already pretty expensive and not the pinnacle of cooling tech any more. If they went up in price, I think people would just go with a different brand. Like Thermalright, which "borrows" from Noctua's designs but are a fraction of the cost. You can even get them in that ugly brown and off white color scheme.
Getting Doom to run on just 3 coolers would be pretty awesome, tho.