A commonly asked question, a great resource to start with you can find here.
If you have two people you should use solid rivets after careful measuring to make sure you have the right length. If you have one person you should use blind rivets. If you are seriously considering bolts, maybe you should go with the blind rivets. They are easy and fun. Saving money on tools is good but don't skimp on the skin of your bus. After you insulate it will be very hard to get to and the cost of your tool and rivets and sheet metal are not much different than the cost of bolts and sheet metal.