It is hard to predict how any future dog will get along with another dog, but I would always suggest opposite sex to be on the safe(er) side. My female dobe and my male bully boy got along great, but that was mostly because my boy knew only one reply to her demands; "yes dear." RIP Angie. I recently lost her to cancer.
Well I'd go with a working line breeder, pricier but worth it for temperament, evaluation and guarantees for nose to tail and everything in between. A good breeder will match you with the right pup for your family, lifestyle, energy level of current dog and anything else you are looking for.