Good point.
A pleasure horse isn't going to do well at speed events and vice versa. If you have your own horse, you may have to try a few things to see what he/she is good at.
Show outfits depend solely on the discipline you show under. A WP outfit is infinitely more elaborate and expensive than a Ranch outfit.
As far as finding a good trainer, going to local open shows is a good way to find local trainers.
If you are interested in showing in the western discipline, you need to first find someone to mentor or train you.
Go to a few local open shows. Watch all of the western classes. Make a decision as to which classes interest you the most.
Talk to people that you see competing in those...