I think the 9.7:1 doesn't give you enough gearing. The bot may have a hard time starting/stopping because the torque is low. I'd suggest a 25:1 or higher gearing instead.
You also need a motor controller of some kind. A "dual motor controller" or "dual motor shield" would be best, as it's easier to hook up, but a "single motor controller" times two also works. Just make sure it's rated for more than the voltage you use and more than the current draw (stall amp rating) of your motors.
When it comes to chassis, if you're not getting a kit (like the 3pi, Zumo, or another such kit) then the "best" chassis is simply a plastic plate (or plywood) that you can screw things into. Get mounting brackets for the motors to screw them into the plate, and make sure you screw them in straight!
Finally, with only two wheels, you will need some kind of follower wheel/caster. Unless the bot is specifically made to balance on two wheels like a Segway, in which case you will need an IMU of some sort (accelerometer and gyro, perhaps with compass.)