[77Birdman]

I would welcome the cats (and im a dog person) to keep the mice under control. Ventilation underneath is not a bad idea, it will keep the rot at bay, but you could install something solid at the perimeter and put in a couple of small screened vents at each end. Then again I have a tight attached garage with good seals at the doors and the mice and squirrels still find their way inside! A good easy ceiling is foil covered foam board that we use to use for ext. sheathing. It cuts easy, light and easy to install, adds a bit of insulation and some reflectivity.