I would expect all windows to be double glazed to get energy rating. The big costs can be earthworks and service connections. Then access to trades. You really need to approach some builders that work in your area for quotes. Everything else is a guess

You seem to have put some good thought into minimising costs where you can. Trade labour is very expensive and not likely to drop much in the future. You will also have the hurdle that a custom plan with specific selections as you’ve chosen is always likely to be more expensive than a volume plan over the lifecycle of the build. Eg even at the start your plans and engineering will cost more for a one off build compared to a copy/paste from a volume builder. I would talk to a local builder, pay them for their time and ask them their thoughts on local conditions and a ballpark of costs.

In Perth, yes you can. There are a number of builders who do “under $350k” builds. As others have said, contact builders in your area and get the correct info for your area.

This design in Perth by a well respected volume builder is 133.19sqm and is under $300k. Noting it includes a single garage, which you do not want, therefore you would delete and have even more living area available. If this is your final home then I would suggest you have the main bedroom downstairs for future proofing for old age stair issues.