I know your show is really close but if you don't have the pants and chaps yet I would suggest calling up everyone you know that might have some that you could borrow or buy. Also look around to see if anyone is having any used tack sales, a great place to find good deals on show clothing! :) If you really have to, of course you can take a day and go out shopping for them.
What you have so far looks good to me, I really like the shirt ;)
One thing I've learned working with horses: You can FORCE a horse to do something on a tight rein and he might do what you ask or you can TEACH a horse to do something on a loose rein and he will do it willingly with his heart.