Once you have decided and booked your puppy, you may wish to acquire suitable bedding, feeding items (check with your breeder what food to buy) and grooming equipment. Most breeders will provide you with a 'starter pack'. You may wish to buy a collapsible crate (minimum size 36" x 24" 27") for travelling, as sleeping quarters and for toilet training.
Take someone with you when collecting your puppy and plenty of old towels in case you have a travel sick puppy. The puppy will travel better if on somone's lap.
Make sure, when paying for the puppy, that you have received all the documents including the Kennel Club registration papers. Check the puppy is insured for the first few weeks before you leave.
Book an appointment with your vet for a general check up and advice on inoculation etc. when you get home. Let your breeder know how the puppy has settled within a few days. Keep in touch.