Thanks all for your advice!
I am a little weary too of letting Kate have access to outside when we're not home (scared she'd be stolen, seeing as she's so darn friendly)! We are still thinking about it, but the good thing is, we are getting a really good gate lock, and our fence is 6 ft high. Also, our neighbour doesn't work and is home everyday, she could totally keep an eye out for anything 'suspicious'.
But, I'm still scared to do it! Regardless, the door would be good for evenings and weekends, as Kate wants out a lot!
Anyone recommend any dog door brands? Or are they all pretty much the same?
Thanks again everyone