Yes, agree with pp. Puppy absolutely must be secured properly in the car, preferably in a puppy sized travelling crate.
A loose puppy could be very badly injured or killed in the event of even a small shunt in a car.
It's illegal to travel in a car with an unrestrained dog for a very good reason.
The puppy is going to have to be restrained when she travels in your car anyway, so, just go with whatever method you're planning to use in future.
Get a portable dog bowl and take water with you. Or you can buy water bottles for dogs from any pet shop that have a little tray attached that the dog can drink out of.
Puppy pads for the journey on the bottom of the crate and take extras in case they need to be changed.
A few treats perhaps, although, giving a puppy treats during a long car journey might result in puppy being sick, so perhaps not a good idea.
Have you discussed the journey home with the breeder. I would have thought they would able to advise you.