Chances are the technique you use for the mask will be dependent on the technique you choose to make the armor. That way you will save on materials and not need to learn extra techniques.
Then, the only remaining question would be whether or not you want to try to articulate the mouth. To do that you would make the mandible separate from the face. Then you would adhere the mandible to your jaw.
If doing a psycho perfectionist costume, you would cast the pieces out of foam latex, slushcast latex, silicone rubber, or possibly polyurethane. You could potentially skip making a lifecast of your head since the face is more rigid, and not so much expression needs to show through.