Jace,
I've built medieval replica armor in the past. You could probaly use 18 Ga. (20 ga.could probably work too)sheet stock . It has enough mass to not be fragile and should be light enough that it wont be a pain in arse to wear. Depending on how your planning on joining it will determine alot. If youre going to weld it try TIG(tungsten inert gas)welding with lots of heat barrier to keep the parts from warping. There was a product out a while ago called Everdur(I think its a brand name , I cant remember) that we use to use to braze (with a TIG torch) sheet metal. It worked geat and used a lot less heat than welding. If your going to Oxy-Acet weld then be prepared to ding out lots of warpage.
Ive started patterning the chest plate for some steel MC armor but thats on the back burner till I get my suit and my kids done.
BTW a GIANT thanks to Sean for the helmet I got from him, fits me real snug but fits my daughter near perfect.
Harbinger