Adrian,
Buying brake parts second hand may not be the best idea... Except maybe for the rotor.
The 15 mm master cylinder is what my bike had with two rotors and Ate caliper from Munchen. It stopped the bike but not had the brake feeling the current bikes have.
So if I where you, I'll buy the second rotor,caliper, and assorted brake lines and keep the 15 mm master cylinder and see how it goes.
Those parts are not eternal. So when time come, you'll can upgrade for a 13/14 mm master.
Hope this helps
Of course I'm not the buyer... So .....