I would suggest, if you can, to go to exactly the type of show you are planning on bringing the horse to and watching. Watch all the classes that strike your interest, and make a list of things to be looking for. I know that in my showing experience, every circuit, breed, event, etc is VERY different!
Example: I went from showing in walk trot in the zone 4 APHA circuit when I was a kid, where saddles were $1400, you had to be dressed to the nines, your horse had to look like they had been groomed by Epona herself, etc... to a 4-H show circuit where the tack rarely had silver, you were expected to wear riding breeches or jeans and a plain shirt, your horse didn't really have to be groomed THAT well if you didn't have the time.. etc.
So, honestly if you can, go to shows and experience it first hand before you try to jump on in it!
Proud owner of 13yr old Paint Gelding, Bali! <3