All of them, Aluminium is 2.49 USD per KG, steel is like 4 times cheaper.

We use plastics because they can be moulded into any shape in seconds. Aluminium is used for premium products instead of steel because it's easier to manufacture into shape - you can cut an aluminium beam as large as your arm with a hacksaw in a minute or two. If that was structural steel, you would batrely make a dent in hours. This reflects on wear of cutting and shaping equipment in factories too.

Stainless steel is the king of materials, but a bitch to work with, that's why you only ever see simple shapes made out of it.

There really aren't any sadly. Maybe cast magnesium, like in sewing machines. It'd be better if the printer was built out of some bio-polymer plastic so it just fell apart after 20 years. But we're rapidly getting to a paperless society so give it 10-20 years it'll be less of an issue.