Idk I only do breed shoes full of glitzy outfits. But maybe roach this mane. It looks more appealing.

(I do halter all the time)
Yes, his mane is fine. Be sure to clip both of their bridle paths, fetlock hairs, whiskers, and insides of the ears, and coronet bands. Be sure they're brushed, and get all grass/hay out of tails. Put some shine spray on their coats if you want as well.
A plain leather halter is perfectly acceptable. Just make sure it's well-fitted and isn't dirty.
You should wear a hat, and a belt is good too. Miss me's and a plaid button up shirt are great attire- just make sure the shirt is longsleeved and the sleeves are rolled down and it's tucked in.
Shows don't usually require proof of shots, just a current negative Coggins and you're good to go.
PM me if you want any more tips, this is already turning into a dissertation, haha