Definitely use 6 motors. As for the speed, visit this page by @Xenon27 called “catalogue of drive gearings” to help you choose a speed. (Around 450-480 RPM on 3.25 inch wheels is the usual). If you need some practice and experience before you design your own drivetrain, start by building the hero bot, and then use the engineering design process to continue innovating and adding your own features to the robot.