I use 1/8" sheet to cut the blanks for my butt pieces. The edges are thinned down in forging. I have used thicker stock, but I think 1/8" is good if you don't work it too much. I know some people use thinner stock, but I like to have a good edge to fit together, and I like a little heft to the piece.
As to trigger guards, I usually forge the entire guard from an 8" length of 1/2" square wrought iron. I used to use 1/8"x1/2" for the grip rail before I started forge welding the entire guard.