I have a Jack Russell. Friendly with people and other dogs (as long as they accept that he is top dog), brave, intelligent and agile. Makes a noise when strange people/animals are about, but not otherwise. If you are a furry animal of whatever size, and you're not supposed to be there, he is your worst nightmare. He's a small dog, but he doesn't know that, and his sheer attitude is enough to deter most things. He chased an adult fox for half a mile the other day, and rabbits, rats and squirrels are mostly doomed if he sees/smells them. I got him as a puppy, and he curls up to sleep with my three cats at night (the cats having established the pecking order very early on). Like Border Collies, they need a job. Olly (my dog) feels his job is 'assassin'. Throw something and say 'fetch', and he looks disdainfully as if to say 'do I LOOK like a retriever???' He has staff to fetch things; we are currently fostering a labrador, and he brings the bodies back.(for Bee; he is a huge, slim, leggy dog from a long line of working, not show dogs).
Good luck with the search, Bobert.
Tim