Dishwashers use only hot. Some dishwashers will heat water, but as far as I know, they only boost the temperature up to optimal. The dishwasher I own has a button called temp sense which when activated, measures water temp and turns on the heating element. Dishwashers are pretty easy to plumb even in a house already built without one. Probably the only prep, would be to split the Hot water and install a valve for the dishwasher. The drain would be easy to install after the fact.