I don't know anything about lathes is the short answer - but I'm learning. I thought that it would be a good project to build a gingery-type lathe but have subsequently read a fair bit of adverse feedback in relation to chatter, torsion of the bed, etc. I essentially wanted to build something that I could machine up the occasional piece for generic projects.The nice thing about Gingery's design is that it more-or-less can be used to build itself. But that was based on being able to acquire cold rolled steel - and CRS seems to be a problem in Australia.
I was also looking at the `concrete lathe' as another option but this is a much larger machine. It has some nice features but, again, I think that availability of material at a reasonable cost would be an issue.
As for type of metal - I usually cast Aluminium, ZAMAC or pot metal. But steel, bronze and brass are also possibilities (with all of the machining difficulties that the latter imply).
I expect the alternative to CRS is to buy mild steel and have it milled to spec or buy 1020 `bright steel' that is close to spec and modify the patterns for the aluminium castings accordingly.