Buying robots is great, but building your own can be more fun. If you know your Arduino, there is no reason you shouldn’t try building your own robot. This Arduino controlled autonomous tank seems like a fun project to try. As Gallifray_Todd mentions on Instructables, you will need a tank chassis, robotic arm kit, Arduino, a Bluetooth module, and a few other parts to get started:
More information about this Arduino project is available here.