I think that guessing at components isn't going to work for this one. You need to do some arithmetic with Watts, Joules, heat dissipation and work out how much heat you want/can get shut of then buy the appropriate components.

I can't work out what you want to water cool or why you want to water cool it to begin with so I can't offer you any advice on it.

Makes a lot more sense to me to put either Nema23 or Nema 34 steppers on it and power them with some reasonable current using TB6560 or similar than to bake the Nema 17. It isn't just heat that kills things, they are designed to work within certain constraints and once you start going outside that then mechanical/electrical failure will occur.

There is a huge difference between quality steppers and the ones that these machines tend to have. I good set of German/US/UK made steppers will work so much better than the ones form China but they will easily cost you ten times as much.

Horses for courses as they say.