There are two main parts in the project: one is the eye tracking, and the other one is the animatronic eyes. Both have to match the requirements and restrictions of the project, like energy, weight, etc.
The tracking problem has a software part and a hardware part, which depends on the phisics of light (in our case IR light) and how it affects the surface of the materials being viewed by the image sensor.
The animatronics problem is all about designing a small-as-possible 3D printing friendly animatronic eyes that can be attached or customized to the user's needs.
I will start building from the two experiences that inspired this project, first my experimentation with arduino and the omnivision ov7670 sensor module, which I've published in my blog (http://therandomlab.blogspot.com.es/2016/06/arduvision-ii-ov7670-fifo-module-and.html) :
Then, regarding the eyes, I've played before with the 3D printer, designing these pair of eyes for another project: