YOU will need to build this:
HC-SR04 Ultrasonic sensor
Battery holder for 6 AA batteries
6 AA batteries
Arduino UNO (I use UNO R2)
Motor Shield for Arduino (L293)
2 gear motors with wheels
4 male-to-male cables
4 female-to-female cables
few cable ties
I buy all components for this rover on ebay.
First you will need 2pcs Plastic Tire Wheel + DC 6V Gear Motor 48:1.
I solder 2 male-to-male cables and 104 capacitors.
I used slim CD box as a chassis, I made 2 holes on each side of CD box and then fixed gear motors with cable tie.