A ring is not just about one diamond, except in case of a solitaire ring. You can choose a big diamond of any cut shape and carat as the centerpiece according to your budget, and consider adding side stones or halo accents to it as per your liking. If you prefer a cheaper alternative, you can avoid opting for a big centerpiece and buy accented ring instead.
A platinum band will be the best band choice as it is more durable than any other metal but very expensive. For a better and affordable choice, pick a white gold or yellow gold band. Choose the band metal keeping in mind that the band color must blend well with the center stone color. For instance, a stone with visible yellow tint is better paired with a yellow gold band.