Who ever is quoting on your windows, get them to run quotes on glass of various thicknesses. We ended up with 11mm glass in our stackers because the price difference between the thinner glass was negligible....we were really surprised.

They look like steel doors but after a quick Google search nothing came up in Qld, only Melbourne and Sydney. I guess you'd already done this too hence your question. You could always order from interstate or maybe use aluminium instead. Maybe try a local heritage architect as they may have a local contact.

I'd say it's an easy min. $250K minimum flagfall on a relatively basic/not fancy spec/fit-out in a super good package deal. You're essentially renovating an entire house including moving plumbing, walls replastering, new floors throughout etc....not to mention lights, rewiring etc etc, it goes on and on.......This scope would still cost a builder themselves $150K, not factoring in their own time/labour, paying cash for sub-trades and getting super deals on all materials....and then you could only really potentially roll this type of operation out if you actually have the money in the bank. If you need to borrow then you'd need a building contract which then brings in market rates, profit margins and GST. Take profit and GST out from $150K and there's barely enough left to cover materials alone, when the labour/materials ratio (of a construction cost) these days, particular for renovations is labour being the most significant cost involved.............you can fine tune and perfect a floor plan like you have (which I reckon is pretty good as a plan), but when push comes to shove it always comes back to budget and costs, which is why we always are encouraging people to utilise design professionals who manage the challenge of designing within budget limits. If you separate budget/costs from the design process (rather than integrate it) more often than not it will leads to disappointment and misalignment of your expectations of what is realistic.

If it is bare particleboard then my short answer is probably no, it is not worth paying double for better quality doors. Like I said, I haven't seen unfinished particle board inside cupboards in a long time. If it is laminated particleboard then it might be worth it. Like The Joinery Store said, laminated interiors are typical. Plywood is not unheard of (and I think it is more common in some other countries) but it would be unusual if you did not specifically request it. Just to be clear, the cupboard has been built and has laminate doors, and you are getting quotes to change the doors? For what reason are you thinking of changing them? What do you want the doors to look like from the outside? Do you want them to match any other doors or features in the room which are timber or poly?