If I want to build an MVP, would you advise me to go for a PHP clone script or a WordPress clone theme? I know I have to develop something from scratch eventually, but this is for demo use in the initial stages. I am not a developer.
Well this totally depends what your MVP is? What's the functionality? I'd suggest something like http://www.squarespace.com/ our designers with very little technical knowledge have had great success with this.
Another option if it's just for demos/feedback is to use something like http://www.invisionapp.com/ for rapid prototyping.