The Yoshi, You'll unlock your next part at 1200 coins. Once you reach 1000 coins, the increment goes up to 200. And once you reach 2000, then they go up to 500 intervals until 5000 coins. Afterwards, the only remaining parts are unlocked at 10,000 and 20,000 coins.
Well, the Soda Jet is the best kart, with the Bumble V, Zucchini and Bruiser close behind. For tires, Sponge, Wooden, Slim and Mushroom are the best, and any non default glider is the best.