Do Cairn Terriers make good pets.?

My wife and 15 year-old daughter called me from a pet store yesterday and wanted to bring, "the cutest puppy" on the planet home - a Cairn terrier. I said how much, and my daughter answered, "it's $200 off." 1. How do you ensure that this, or any dog at a pet store, is not the product of a "puppy mill?" 2. Is there really an advantage to having a dog, a Cairn terrier in this case, "with papers." 3. The only dog we've ever had that liked me was a mixed breed rescued terrier (Wags, a 20 pound friend of the highest order); 4.) I'm a jogger and the dogs that give me the most trouble are the small ones. How will a Cairn do? What is their disposition regarding aggressiveness. I realize training is a big factor, too; 5) What should you look for in a Cairn puppy to decide if he (and this dog is a he?) will be a credit to his breed, and gender? 6) Finally, how do Cairn terriers differ from other types of terriers?