Not sure I've read this right, are you saying your thinking of Collie crosses because A, your only interested in Rabbits? Which is fare enough. A good Collie cross will do you proud on the humble bunny and B because they won't have it in them to tackle Deer. I can assure that the do have the necessary minerals to take Deer as I'm sure many on this site will testify.
If you don't mind me asking why are you not considering a 1/2 bred? 1/2 breds have plenty of pace to cope with rabbits, have more stamina than 3/4 breds and are generally much tougher physically.
i know were you are