Hunt coats are not appropriate for dressage, shadbellies are only legal at PSG and above. The dressage coat you have selected is appropriate, just make sure it fits!!! It is so worth it to take it to a tailor and have it made to fit you perfectly.
For the boots, it doesn't matter really what kind you get as long as they fit you. As you move up the levels you will want to invest in a pair of Cavallo, Petrie, Konig or other real dress boot. The stiff boot and high quality leather looks very sharp, and I find they improve the look of the leg.
For a helmet, a plain velvet helmet is appropriate. The ones with the front vents are butt ugly and made for the jumper/hunter ring. In dressage they look tacky. I have an plain velvet IRH with a dial that I adore and although it still has the same safety features as the other helmets with the front vent, and adequate ventilation without looking like ass, it was much less expensive. Save the money from your helmet for a top hat when you are older.
Breeches are personal taste. Personally, I dislike both those breeches. If you do decide to purchase either, you are going to want to look into purchasing white underwear as well because even if you wear a light pink or grey, everyone watching will know it. I ride only in Ariat, Arista and RTS because they are the highest quality and I can wear bright pink undies if I'm in a pinch and no one will be the wiser.
For the stock tie, make sure you are also getting a pin. There are really cute ones available in tack shops, and you can get a steal if you go to an antique shop and pick up an old pin. Plus no one will have it!
For shirts, I prefer short sleeves. I find it gets very hot, even a few hundred miles north of you! Get white, or any pastel colour.
And make sure you are purchasing a pair of white gloves, and if you have long hair a show bow.