A good contract will ensure that both the breeder and the buyer are protected. It should spell out the responsibilities of both parties, as well as the expectations and guarantees that are being made. The contract should include information on the dog's health, vaccination history, and any microchip information.
Be aware that most contracts in the dog world are not legally enforceable, especially the ones that are micromanaging. Legally, dogs are property. As a result, do not rely on contracts to ``force'' the behavior that you want. Instead....
Remember, dog contracts often fail as legally binding documents, so a compatible buyer is much more likely to result in success. Be aware that contracts that create long-term relationships such as co-ownership, breeding arrangements, show requirements, etc.
In the painful event of a serious dispute, a court is more likely to enforce terms that are clearly spelled out as essential to the Breeder's agreement to sell the puppy in the first place. Not everything is a Deal Breaker - Remember, if the contract is too overbearing and unreasonable, it likely won't be enforceable.
Deposits are completely normal. Some breeders even ask for them in order to be put on the waitlist. They help the breeders to know who is serious and to know that the puppies definitely have a home.