Before installing storage units, lay down cinder blocks, concrete piers, a concrete slab, or at least crushed stone. But don't stop there. Make sure the supports underlying the floor are made of pressure-treated wood.
Make sure the entry to the unit is wide enough to accommodate your largest piece of equipment, such as a gas snowblower, with room to spare. Many outdoor storage buildings that are at least 8 feet x 10 feet come with double doors, which usually eliminates this concern.