You will want to have multiples of your show clothing AND keep the extras in your trailer/changing room areas, just in case of ripping.
You cannot go wrong with starting with white
Buy your show clothing
as cheaply as possible. ONLY invest in more expensive if you really become serious. Right now I would spend the most on your every day schooling clothes, and boots.
State Line Tack has better prices. English Show Clothing - Coats, Shirts & More - Statelinetack.com
Dover is a little high end, but I love their stuff.
Now, if you're talking tack, that's where I'd spend money, on good quality show saddles/bridles.
You wash your show clothes and polish the show boots right after a show and store them in bags to keep them clean.
Do NOT use your show clothes/tack to school in or they will LOOK like you did. =D