Okay the only part I can comment on is the plates. If you do use foamboard I would recommend a primer of a H20 Latex paint and then use whatever metallic paint you prefer but that h20 keeps the foam inside the foam board from rotting,melting, or generally mucking up. Also if you've got the money for wonderflex or heavier plastics and can spend time working them then do that! My foamboard cosplay is me staying in a super cheap budget. So far I've built most of the armor for my set for less than the cost of a pair of shoes for another cosplay! ^_^
I wish you luck!