Most of the atx power supplies deliver 5V at ~25A and 12V at ~17A. Why you need a load resistor? doesn't your stepper drivers have current limiting? Are you planning to use multiple ps units parallel to get 24 or more volts?
i have used a single atx supply to wire up all the logic circuits, cooling fans, relays, lights etc and never got a issue. The best thing about atx is that they have built-in short circuit protection , so you may save your expensive electronics from burning.