There are a number of things to consider;
Will you be hand stitching or machine stitching? If by machine, what is the thickest material can you currently sew with it? Factor in the number of pleats and waistband....have a good supply of needles on hand
What style of kilt? traditional, utility, Roman. Depending on the style will determine the end weight of the kilt, which shall determine what weight of leather and how much you will be required to use. As for the kind, the sky's the limit, ostrich, moose, lamb, cow, bear, in the end it's up to you and what you're comfortable with working.
Good luck