replacing decking wood. Gaps between the planks

I've a rasied Queenslander, and it's built underneath with a rumpus room and a garage.

At the front I've a deck, and I'd say that about the front 1/4 is just a raised deck, where as the remaining 3/4's serves as the roof to the rumpus room and the garage. it is entirely covered with a roof, but rain will blow in around the edges. I don't believe it is ever in direct sunlight.

The current decking is painted wood, but it is rotten in a few places, mainly in the front 1/4, as this is where the rain would get blown in. Hence the reason to start replacing it.

But there is a difference in how the wood has been laid. For the back 3/4's, where it serves as the roof, the planks are very close together, with no gaps between each plan, so rather like floorboards. I'm not sure if the gaps have been filled with anything, but I'd suspect that the paint acts as a seal.

But for the front 1/4, there are gaps between each plank, which would be to let any rain water that blows in to escape.

I'd really like to replace the entire deck and this time have some kind of natural 'oiled wood finish' rather than pained wood.

Could I put down something like merbau and then put it done so there are no gaps between each of the planks across the entire deck so that it looks even, bearing in mind that it's going to expand and contract.

Any thoughts on how I could the seal the gaps, and still maintain the 'natural look' of the wood?

is there a better wood to use in regards to the expanstion/contraction properties? Cost is an issue, so I'm trying to use a cheaper wood option.

Any other thoughts/advice?