What experience do you have? What kind of dogs have you raised and have you done any sort of competition before?
Just off the top:
-Shar-peis are notorious for serious skin conditions and allergies
-Boerboels, while relatively low energy in my experience, need expert socialization and training. Otherwise they can be extremely aggressive and have a tendency to take a situation into their own hands which can end badly. Even with proper socialization, they have a tendency to be a bit stand-offish to new people and things. I would not recommend this one unless you are VERY experienced with large, natural guarders.
-Newfies and Leos both tend to be pretty laid back. They both can have pretty short lifespans and they have the general health problems of large breeds, namely hips and eyes.