Welcome to the 405th! It's totally understandable that a spartan build can be overwhelming. Some advice would be to start with something simpler, like a boot, or a knee cap. That way you can practice your foamsmithing on these easier parts, and if you feel the need to remake them later on, it won't be so hard to do so.
1/2" foam mats might work for larger sections such as the chestplate, but for areas with higher curvature and small details, you may want to get smaller thicknesses. I'd recommend 0.27" (sorry, I use metric and we use decimals so this has been rounded up) /6mm foam when you need something thick but flexible, and 0.12"/3mm foam for details.
Is this your first time foamsmithing, or first time on a project of this scale?