Back in 2004 we constructed a second floor, including kitchen-corner, living/sleeping space, of abt 6 x 8 mts of laminated wood, supported by a frame of wooden beams: self supporting floor, walls, roof (wooden sheets with on top corrugated plastic sheets as a rain protected layer).
Partly painted, more for "looks" than for protection.
We remodeled the first floor (originally made of adobe) in brick and concrete with an outdoor (space-saving and privacy) concrete staircase to the second floor.
Situated in Chorrillos/Lima so quite close to the sea....
It lasted without any problem (moisture, "rain", insects and other threats) until the end of 2007 when a blasting fire destroyed the whole block of houses including that of my sister in law.
However, if I would have to do it again, I would probably use for the second floor drywall (double layer, with space between the two layers, for insulation purposes and to hide electricity wiring, et cetera).
However, I would not suggest the outer walls of the first floor to be made of wood or drywall, because of the vulnerability of the material.
For the outer walls of the first floor I would prefer a brick, a concrete, a mixed brick/concrete or similar construction....
Please keep us informed of your choices/experiences!