You will need several mechanical and electrical components for this project apart from the Arduino Yún. The first set of components is for the robot itself. You basically need three things: a robot base or chassis that will support all the components, two DC motors with wheels so the robot can move around, and at least one ultrasonic sensor in front of the robot. We used a mobile robot kit from DFRobot (http://www.dfrobot.com/) that you can see in the following image:
The kit is called the 2 Wheels miniQ Balancing Robot chassis and costs $32.20 at the time of writing this book. Of course, you don't need this kit specifically to build this project. As long as you have a kit that includes the three kinds of components we mentioned before, you are probably good to go on this project.
For the motors, note that the circuit we used in the motor shield can handle up to 12V DC, so use motors that are made to work at a voltage under 12V. Also, use motors...