Have someone in the backseat with the dog to keep him company. Have a bowl of water (preferably the same water that he is drinking at the breeders, you can bring a plastic bottle and fill it up at their house). If you can, stay at the breeders until the puppies are done playing together and leave at the nap time. That will give you a bit of time for the drive back home. Prepare the backseat of your car for potential urine and puke which may happen.
You will need a small collar for your dog along with a leash. I assume that your house is prepared for the dog? Did you stock up on supplies like a crate, dog bed, puppy food, toys, scott towels to pick up eventual feces. A nice thing that people don't think of is urine scent remover, when you use that after the dog urinates or poops on your floor, it will remove the smell and the dog will not be as tempted to go to the same spot.
Oh, and prepare yourself for a heck of a first night. If you are going to directly crate train your dog, he will bark and scream all night. Remember to only let the dog out of his crate when he is not making any noise. But you may opt to leave crate training for another day, which you should know that puppies can only hold their urine for about 4 hours maximum. So you will have to get up several times a night to let puppy outside.
Also, you should schedule an immediate trip to the veterinarian to make sure everything is fine with the puppy. Most breeders will only give you your money back if you bring the puppy back within days after you picked him up.
That is all I can think of right now.. Good luck with your new best friend.
