For the metal looking like plastic : darken your diffuse, put the scratches in your specular, I recommend using a gloss map too, to give the shininess that "dirty" non-uniform metal diffusion.
Most shields wouldn't be constructed like that without a trim around the outer edges. The typical trim exists for a reason namely to reinforce the strength of the shield so that it wouldn't break down as easily. If you're dead set on the design I'd at least make the edges very shattered and worn (with big chunks of wood…