I agree with this too. And don't be like some people and mistreat your barn cat. Cats turn into buttholes because of abuse in my experience. Treat them good and they'll clean out mice like crazy.
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This is exactly right. The only way you have a bleeding heart for mice is if you've never dealt with them.
To the OP, as someone said earlier, having a slab inside will be the first big step. Then investing in good solid garage doors and man doors. Then insulating and finishing off the inside...