If your using a 9" motor why are you only going to use 72 volts? I did my first conversion with 72 volts and I guarantee you that you will want a controller that will pump out over 500 amps. My controller pumped out 550 amps. Most controllers in the 72 volt range just don't have enough poop to do you much good. You will want more right away, guaranteed. So to bypass that step and save yourself some money why not go up to 96 or even 120 volts. The Synkromotive controller is a well designed and robust controller with a decent price. I'd steer clear of Alltrax and Kelly unless you have a golf cart your using. So you choices are Curtis, Synkromotive, Soliton1 or Jr., WarpDrive. All being decent controllers and all being able to run at lower voltages than the max. I ran 120 volts with a 156 volt Synkromotive controller just fine. The Synkro also puts out good amperage and the most for the price range. Air cooled and its pretty near impossible to overheat it. Its small and lightweight and very programmable.