By way of warning, I’m less fashion-conscious than many, but around here I’d go with light tan breeches, either tall boots or boots+halfchaps that match, and a white shirt with a collar. (Polo is fine.) Black gloves if you want 'em. Black helmet. Any English-style tack that fits you and the horse. (Note bitting restrictions in the rules, though.)
Reality vs rules: the rules say “conservative color” for the shirt, but white is expected. “Light color or white” for the breeches, but around here light tan is expected; I’ve heard white breeches called more appropriate for professionals. (White is much more the thing at pure dressage shows.) The rules say black or dark blue helmet, but that gets routinely ignored in favor of bling and nobody gets called on it that I’ve ever heard of; despite that I’d still opt for black if you have it, or anything that fits your head otherwise. Jackets are not required below Prelim, but around here I’d say most people at recognized shows wear them; at schooling shows, mostly not. (Add neckwear and a different shirt if you want the jacket.)
Nobody who matters cares if you have half-chaps or tall boots. Same for style of horse tack. If you’re matching, clean, and tidy, don’t worry about the turnout any more and stick to riding the best test you can. Since you’re talking about recognized vs not I’m assuming lower levels, but you don’t need a dressage (or jumping, or whatever) saddle until not having one is impacting your riding.
