Over here in the US cement fiber sheets and planks have been available for several years. I'd think
it would be available somewhere in SA. Look for the name Hardiboard, Hardiplank or do a search with some combination of cement/fiber/panel/plank/board. The cost is around 50% more than plywood over here.
This sheet/panel product is formed in 4x8 sheets, 1/2" thick, with a face appearance similar to wood
grain. The planks also have the wood grain. I've not worked with the planks in some time, I believe they are about 8" wide, 16' long. The stuff is impervious to water
, extremely fire resistant, wont rot, and you'd need a pretty big bug to do damage to it. For the environment you described, this material would serve your needs very well. Paint as you would wood, it should
offer all the protection from humidity and moisture you ask for.
The product looks great and holds up to the elements. There is a disadvantage-it does not hold up well to impact forces. It will take a baseball as long as it is not thrown by superman. It will hold up fine to stone thrown by a mower. If you try real hard, you can drive a stud through it, but you may need a running start. I've used the stuff before. Sheathing the structure with OSB or plywood followed by a moisture barrier will make the wall practically indestructible. Connect with nails or screws. Caulk the seams.
Have a look at Durock. Same sort of material, but without the wood grain face. This would be suitable for floors or walls which would be covered with tile. I've used it in dozens of bathrooms as a wall backing around the tub/shower. As a backing for tile, Durock cant be beat. Also have a look at Hardibackerboard.
Tile would hold up for you, but can also get pricey.
This is a side of my brother's place being redone with the hardiplanks as clapboards. The trim around the windows and corners is made out of the same stuff. Cuts with a skillsaw.