Is it going to be fully scratch built? What kind of material will you be using for this scratch built toy? Usually when someone scratch builds something, it's usually more of a model since model grade the plastics are easier to work with. Where would one find sheets of toy quality ABS, PVC, etc? I guess if you have some CAD skills and access to a rapid prototyping machine, a machine to make molds, and a plastic injection machine, you can pull of a scratch built toy.