Don't block off the soffits. When you insulate you need those little holes open for air flow to the attic. For starters start buying a few sheets of drywall each week. For $5.00 apiece you can pick up some here and there and hang them on the ceiling. This will keep the heat down instead of going up. Although it will go thru the drywall eventually it will keep it warmer. The cars will eventually dryoff with heat. I park rain soaked vehicles in mine all the time and have never had tools rust yet. But my garage is a dry garage meaning the floor never sweats nor has moisture ever been a problem even on the rainiest of days. You will have to isolate where you are getting moisture from. If you are going to put a slab down, either put down visqueen first or put down foam insulation board and pour the concrete over this. Most moisture will come from the ground up. Once you start putting in insulation it will start taking care of the moisture. When you start mixing the warm air with the cool air is when you get your condensation. And that seems to rust tools faster than a dripping wet car. Condensation seems to collect on anything that is cold, like tools.