Aussies are excellent dogs to have, matter of fact they are my favorite, least favorite...my wife's chihuahua. However, when I was younger we had one that did kill some of our chickens, we cured it by tying a dead chicken to its collar on a 3 - 4' piece of rope. If you get a puppy you can train it not to harm your chickens. My wife also has a sheltie and I trained it not to harm the chickens, matter of fact the sheltie loves to play with the chickens and they get annoyed by him and give him a peck or two and he just shrugs it off.
In regards to the owl, you might consider building an enclosed coop, the chickens will go in it at night (or just before sundown) and that should keep the owl away from them. I am pretty sure that owls are protected just like hawks are eagles so shooting one might get you in some trouble if you are caught...unless you are out in the country and you follow the SSS method. Keep in mind that owls are great to have around for the control of rodents.