I'd recommend finding the dimensions of all the components you will be installing in the enclosure and lay it out on paper (or CAD) to make sure it will fit like you plan. A professionally designed electrical panel would be designed this way. You don't want to be surprised. Also, when laying out the panel, make sure to account for the fact that the enclosure's backplate will be smaller than the enclosure itself.
Material is up to you. Plastic is lighter and easier to cut holes in. Either way, I'd recommend an enclosure with a NEMA 4 or 4X rating.