I havent built walkers before, so cant really comment on that- except that if you are new to arduino and want some fast results, I would start out with two continuous servos and build a differential drive - once you have that + sensors figured out, take the next step.
what I actually meant to say is, if you want camera input, it might make sense to get an ADK board. You can do video processing and wifi communication on the android device and then have the phone talk with your sensors etc. through the adk board.
you will not be able to implement any satisfactory video processing with an arduino.