Hi MWM - A big mill build. I watch with interest. And its a very big mill. So you need to start at the end and research how you are going to mill the rails lands flat and level. This is the crux of that part of the journey no matter what material you choose. Even well made machines have come unstuck with the finish machining so you really need to understand how to do that. It will be a major pain to get it home and find its way off the mark as some people have found. So you need to trial it at the machinists somehow....
Steel is the easiest/stiffest and cheapest build material and is friendly. EG has issues and I think you should read some of the EG threads to get an understanding of its many pit falls. The 20" 500mm Y is getting deep so towards gantry territory. But I'd go moving column to simplify the drive system. Its more complex with a gantry getting two drives to co-operate and be accurate. A horizontal is wicked but harder to set up jobs. So from what I have investigated over the years here's my 2c worth 1) all fabricated steel but you need to commit to stress relief and final machining 2) thin steel plate (laser cut to parts welded) then fill with engineering grout, stay away from EG.. but still have to final machine steel inserts. 3) I would not use steel hollows they limit your design and have various issues. I'd use thick plate bolt/ dowel machined to size and bolt it all together or weld and stress relieve. It will be much bigger then you think it should be to achieve good stiffness.... Good luck on your mission. Peter
addition - the spindle will take up considerable time to sort. Especially if you want to machine steel. So before you design the machine you need to sort the spindle as this will dictate many parts of the machine. Peter