I would like to see more photos of the area to get a better perspective. It looks a very rough area and a lot of work will be involved, including building substantial steps to get up to the sitting area. Quite frankly, I would do away with grass altogether, it won't be easy to mow, especially if you are limiting it to a small area, and put a timber deck in, which could form a viewing platform for the rest of your garden. From there a simple rockery would take care of the natural slope, leaving generous pockets for plants to spill over. Do you need a play area for children at all? Have you factored this into your garden?

Framed corrugated iron shutters left natural would be my preference, colour might be a bit 'try hard' and natural will sit in the environment more easily. Could even make barn doors with it if you prefer that look. The actual barn door fittings are the pricey bits I believe. Our cyclone was made of corrugated iron (by pro shelter builders) and it never ever budged (loud though) so a bit of coastal? wind won't worry it. You don't have much wind going by photo's, all trees seem to be growing upright so a regular build will be fine.

If you do want to put the doors in where the oven is can you not just flip the L shape? So you have a breakfast bar running between the kitchen and dining? Might make your dining much smaller but you'll have the added eating space at the bench. Alternatively, take out the living room wall (to the hallway), close off the doorway into the kitchen and have a galley style kitchen (that's a much bigger job!). Tricky one! Ask everyone that comes to your house what they think - we did this and although we ended up working it out ourselves it was useful to go through all the wild and crazy ideas everyone had.

Looks much neater now.