We did pressed metal splashbacks on a display in a kitchen showroom I used to manage, and as such many of our clients opted for the same look. Powder Coated is definitely the way to go. It stands up really well for durability and is in fact relatively easy to clean compared to many splashbacks. Often people think that glass or stainless steel are the easiest for cleaning, but due to being such a flat reflective surface they show up every splash and if you're going to clean them you really have to clean the full length of splashback. The textured surface of the pressed metal means that it doesn't reflect the same amount of light so you are able to get away with cleaning it less often, and you can also get away with just cleaning the dirty area rather than the full length. The biggest difficulty we found was getting the splashbacks installed as there aren't really any tradesmen who specialize in fitting pressed metal, and those that were willing to cut & fit it were concerned about what would happen if they damaged the paintwork. We had the panels cut to size first before they were powder coated to minimize the risk of chipping the surface.

initial suggestion is about 50 sq m extra floor area and has minimal changes to existing floorplan and doesn't allow for an extra powder room as there are already two toilets in the house although future inclusion of the wiw and ens would allow for some changes but size and costs of the extension will depend on distance to garage, allowed distance to side fences, how the new roof is constructed, materials used and you must assume the existing slab is not good enough unless it was specifically built as a foundation and this is confirmed by a structural engineer ..there also needs to be stairs down from the original house to the extension, a side entry would be useful from the carport and the kitchen may be better at the back of the laundry BTW the dimensions shown on your plan are not consistent (the dining room and kitchen sizes don't match the other rooms) and you need to confirm the correct sizes of every room to help decide about possible floorplans

I have a separate guest toilet without windows using a ceiling extractor fan - not a problem. I also have internal cavity sliding doors to bathrooms and toilets and love how they give extra space and options regarding placement of items. Another location alternative for you may be between the cinema room and the kitchen, depending on what you had planned for that extra space in the kitchen. Two metres is a wide hallway, if that is what it is? Ours is 1.3 which is considered wide (as we need to accommodate a wheelchair which has to turn into doorways). An advantage of a very wide hallway is it gives room for extra furniture placement such as bookshelves, linen or other cupboards if needed. I don't see a linen cupboard or pantry cupboard in your plans. Something else to think about is future proofing - try to cover future possibilities with regard to function, as making structural or fittings changes later can be more expensive, and may come at a time when you can least afford it (e.g. injured, disabled, children, aging or other such people visiting your home). Lives change, sometimes over time, sometimes in an instant, and suddenly the doors are not wide enough (especially the toilet door!), or the walls are not strong enough for a weight-bearing rail, and a host of other things. We put 900cm doors as a minimum and it makes life a lot easier, including moving furniture in and out. Universal Design principles are worth a look. Sadly these considerations are currently considered special or optional, although there are moves to change that, so that builders and architects will consider these things as standard.

as dreamer already suggested a more spacious one bedroom downstairs with small kitchen plus laundry would be more attractive for a couple than two small bedrooms and almost no living space but also depends on if you are aiming for overnight or long term tenants and space leftover from stair well could be useful for enlarging the kitchen with a pantry and upstairs laundry and new entry allows for a study space and master bedrooms suite but you will need to confirm that the old carport is suitable for updating to living quarters but please explain where you think there is space for an extra lounge room apart from downstairs?