It's not heavy enough unfortunately, but it is cavalry twill by the m! So you've done better than I have so far. Needs to be 32oz (900-1000g) so that it sits right on the horse and doesn't flap around showing your legs off!
I thought actually making the jacket and apron skirt and tailoring it (never tailored anything before!) would be the hard bit, bud so far finding the pattern and fabric seems to be causing no end of problems.