Anywhere you'd like! There are a ton of all-inclusive resorts and hotels over the island. We will be staying at Hotel B Unique. If you'd like to stay close to us, Hotel B and Hotel B Unique share amenities (more info under Travel tab).
Classy/casual attire. You're welcome to wear something comfortable like shorts and a t-shirt or a sundress and sandals. You can also dress it up and wear a suit or a cocktail dress. Or show up dressed like a coconut, anything goes!
USD is commonly used in Cozumel. You can pay for almost everything using your card or with USD. If you'd like some pesos, go to one of the bank ATMs on the island and withdraw your money there. Try not to use the ATMs on the street, as their charges are really high.