" For me the idea is to start small but with a bit of class and add on when additional funding becomes available."
What about the Tassie Street Roundhouse?
The concrete floor and tracks are still there.
To start off you don't need a working turntable.
If you could build a steel framed replica of the roundhouse, maybe four stalls to start with and expand it if further exhibits become available. You'd have to build longer stalls than were there originally to fit the Budd car.
It is very close to the business district and next to the Gulf. It would be visible from the highway bridge.
You'd have to get G&W to give you use of the land, but I haven't seen anything move in that part of the yard in decades...
If converted to dual gauge, you could display relevant items from Pichi Richi from time to time.
And insisting that the building looks like the original might reduce the temptation of the architects to get carried away.