I don't know why people think that Toon=Diffuse only. It really doesn't. TF2 for instance uses normal and spec maps to great effect in support of a "toony" look. Anyway, it's your bag, do what'cha like. Though, spec will help A LOT with selling metal. . .
Unity has good support for Alpha/Normal/Spec/Emissive. We're trying to go for a sort of Craig McCracken/Next Level Games toon style, so we're experimenting with having nothing but a diffuse and seeing if that's all we need to achieve the effect. The shotgun texture I have going right now is kind of an experiment, which I'm…