Boiled wool

[Woodsheal]

That boiled wool looks like lovely stuff, but expensive. More affordable period-correct wools are here (check out the broadcloth and worsteds):

http://www.wmboothdraper.com/

I've never noted leather trimmed button holes, etc. on 18th C. garments, leather-lined pockets, yes - especially for carrying musket balls, flints, etc.

Epaulettes - no. A common method used instead: sew a button on the left shoulder of your coat/jacket. Sew a small fabric loop to the top edge of your plaid at the center. When you belt your plaid on, simply fasten it up using the button and loop. Period images show this method (see below).

Pewter or brass buttons are fine, especially the latter. Brass was used alot in the Highlands....



BTW, regarding the Champion's coat cuff in this pic - normally it would be buttoned or folded up, revealing the shirt cuff beneath. It was a common practice for swordsmen to unbutton their cuffs and turn them down to give better protection to their wrists during sword fights....