Acclimating my 7 yr old cat to a new puppy...?

I am a single female living in a "rougher" (read:more affordable) neighborhood. I need a guard dog! Found a great 2 mo old female puppy (doberman mix.) But I already have a 7 yr old female cat. She has lived with dogs before and loved it, and also had a brother until 3 yrs ago. Since then she has been the only pet. But she is very active, and needs a playmate also! Just looking for any tips/advice/warnings for my situation. Thanks!