Thanks y'all, I've used goat hide from a bookbinding supply store, 0.6-0.8mm thick. I see now that I should have been more vigilant about flattening the sinew under the leather, as every tiny ridge detail is visible, even though I filled lots of the dips with little bits of scrap sinew beforehand. You live you learn. It is mostly smoothed out, I don't want to sand right through the leather on account of any small dips. On another bow that is still seasoning I poured leftover glue from sinewing over the back, hopefully this will give a smoother finish.
As for the gold, I was planning on making shell gold, as I'm much more familiar with this process than gilding. I looked up some sources for leaf, I suppose it isn't that expensive at all, especially compared to the cost of everything else that goes into the bow! I'm all stocked on brushes and pigments, as I'm inclined to paint every now and again.
I suppose the priming and under-paint layers will be the same for me since the leather I found was pre-dyed, I will try to match the existing leather color.