I think you might be a bit over ambitious for a project like this... I would start with the basics, like building the $50 robot or getting a Parallax BOEbot. Once you have the basics down, you will have a much better idea on how to proceed and be able to ask more specific questions than "does anyone which micro controller would be best for this project". If all you are trying to do is move a servo motor to move a mouth, you don't even need a microcontroller.