I built my frame from steel angle and the gantry from alu profile. While this works well for most of the stuff I have been doing it is failing to produce good cuts on some of the things I am pushing it to do now.
So the upshot of this is that I am now designing a new one made from steel box section.
For the most part the frame does not have to be perfectly straight, you can always shim it to get things level.
As for milling steel I wouldn't even think about it.