Hey there Wimble,
Welcome to BPO. You will love this group of people. They are great, friendly and very willing to help with advice, opinions and sometimes just knowing they are there and they really care helps. Sounds like you already have some wonderful "Paws" in your life. We really need pics, you know. So, you want a Tibetan Mastiff. I have one we got through American Tibetan Mastiff rescue. He is my dream dog. Don't get me wrong, I love my Wolfhound and my Pyr, but Cody is the dog of my heart. He is not a social dog, not friendly with strangers, will not take treats or biscuits from anyone other than family, etc. He guards my house and me and is my "saint" of a dog. Which breeders are you looking at? Who have you talked to? Welcome to the group, you will love it here.
Kathy