Controllers are optional—in the same way that flying with a joystick instead of just the mouse is optional—but you'll definitely get the best experience if you use controllers. You'll also need at least one base station. The $500 kit with the headset, two controllers, and two base stations is probably a good place to start... especially considering the "Pro" headset comes with no controllers and no base stations, for $300 more!
Just a heads up as well... at our recent FlightSim Expo booth, we were showing off the Vive, Oculus Rift, and a Lenovo WMR headset. Based on user comments, the Lenovo was the one people tended to prefer... and it sells for about $275 on Amazon. (That includes two controllers; no base station sensors are required, since the WMR headsets instead stick a camera on the head-mounted display itself.)
