Prey is used to channel the stress from the defense work. Without prey drive, it is very difficult to successfully relieve that stress to an acceptable level. The dog will always be in conflict and will not realize success. Play drive is used to relieve the stress of obedience work, to increase motivation and drive. Retrieve drive is important also.
Strong nerves are vital.
It is important that the pup not frighten easily. Look at the temperament of the parents. It is possible that strong nerved parents will produce schitzo pups, but it is far rarer that poor nerved parents will produce a strong nerved pup. Observe the mother in particular. The first 6-8 weeks of the pup's life are spent with the mother.