Bless your soul for being so dedicated to that horse you ride :)
Because he/she isn't yours, I don't know that I would spend some money on a saddle for him/her. There are so many things to take into consideration and so much money to be spent that I am not sure I would personally spend money.
Doing research on saddle buying tho is excellent. So much to learn now, get you ready to buy your first saddle for YOUR horse when it happens?
Good Luck with your research :) How to choose the pefect horse saddle