I love getting dressed up for shows! And if we keep on insisting to people that riding is a sport, not just a hobby, then we need to maintain that theme. All sports involve uniforms- even my little brother's Little League softball team has to wear baseball pants and a team shirt to play. It unites them and makes it so that no one individual stands out b/c of their clothes. Instead, you are drawn to their ability. it's the same with riding shows. You can't just let people enter wearing whatever. Heck, if you could most of the people in the US would wear jeans and a t-shirt to jump and do dressage!
Wally, I don't think that it is too make anyone feel bad. It's more to maintain the fact that riding is an organized sport. And all sports have uniforms and their own terms. Homerun, swish, slapshot, dribble, slide, serve- these are terms that people might know, but only b/c they have heard them used in the sport. Riding has its own terms too.
About the clothes, you can buy used stuff, like the jacket. Most tack stores carry used equipment.
http://www.ebay.com often has hunt coats if you can go to a tack store and find your size. Visit different tack stores and the salespeople will be happy to help you. Also, check out different riding catalogues. Most people get white shirts, but I like to add a little color- I have 2 light blue ones and they look great with my navy blue coat. I prefer navy to black or green, but its really just preference. All are the basic colors. They come solid or in different pinstripes- again, preference.
You seem young enough to get away with jodphurs and paddock boots- as long as you aren't really tall for your age. Tall boots are more expensive, and when you are young, you grow out of them faster. You might already have paddock boots that you can use.
Last, if you have a freind you can borrow stuff from, do that. It's much easier of you can but things a piece at a time rather than all at once.
Good luck!