Cammie, I vote for getting a young dog, but research the breeds to make sure you find the one that will be best suited for your lifestyle and your future country living. Make sure it will be good with your rabbit and future chickens ... hey, bring it to poultry auctions!
I agree with oiemommy about adopting from a shelter ... I've adopterd several wonderful pound puppies and I couldn't be happier with them ... and they are adult dogs. There are even pure bred dogs at most shelters. Sometimes the dogs have bad habits that are virtually impossible to break though ... like killing chickens.
I decided that we need a great pyrenees because of their reputation of being excellent poultry and livestock guardian dogs. I actually had a beautiful male great pyrenees about 5 years ago, that I adopted from the Norman Animal Shelter and he was an awesome, wonderful pet, but a pack of wild dogs attacked & killed him when he was approximately a year old. It was horrible. I chased the wild dogs away, but he bled to death ... the vet was unable to save him.