Hi @Libster585,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
I think it is important to note that this is both a large and fairly complex structure that would almost certainly require engineering plans for submission to your local council for approval. The engineering plans needed for council approval would detail the type and calibre of materials required, as well as the footing requirements and tie-down requirements for the roof structure.
Rules vary from place to place, and you would need to check with your local council, but it seems likely, due to the complexity of the structure, that this is something you'd require a licensed builder for.
Unfortunately, all steel decking systems that I am aware of are modular and would have a lead time. There are no steel decking systems that I am aware of that could be purchased off the shelf.
I'd suggest you check out our supplier Steel-Deck, who produce modular steel deck framing that can be customised to suit your needs. This might at least give you an idea of the cost and the materials that are likely required.
Let me know if you have any further questions.
Jacob
