Several things come to-mind. You're in a high-humidity area, the building will probably sweat, and your tools will soon rust, I believe. A heavy-mil poly sheet under the slab will help keep water vapor from moving upwards. Use a good, thick pour and steel mesh w/a thickened perimeter w/at-least a #5 rebar in the thickened edge. Not sure what type of door/entry you plan on having, to keep water from entering the building, put some-sort of drainage channel where your entry is. I think you need to add some method of sealing the building to the slab, to keep water from flowing under the building bottom. Consider mounting the building to a PT 2x4 or 2x6 with some type of waterproof sealant or roll sealant between the PT lumber and the slab. You may need to do something to add to the bottom of whatever entry you have, if you do the PT lumber between the building footer of the walls, and the slab. I don't know what type of metal building you're planning to buy, but a cheap Arrow building may keep the rain off, but is probably not sufficient for long-term storage, and protection from typhoons.