The coldest day here so far was -6F, and we've never lost a barn cat to the cold, not even any frostbit ears. We don't do anything fancy for them, although they do like to hang out in our hay barn. We have the big 5' x 5' round bales, so it makes all kinds of cozy tunnels that they like to hang out in. At 30F, she'll be fine, especially if she's always outdoors so that her body can adjust. As far as spaying goes, make sure to have it done by February 1st if you don't want kittens, as that's when our cats start going into heat for their spring litters.