Lots if things here to talk about but to start with the basics first.
You can do a weldment and be just fine and sand filled or glue and sand is a good idea. You want to make a frame that will allow you to adjust it so you can tram the mill.
If you want to make a lot of things chip management with water cooling or a blower and a path for chips to go is a good idea.
16mm screws are fine and 425 oz steppers are good. The Geckos 3.5A are fine.
One thing to look at with a high speed spindle is your cutting speed. The 425 oz steppers are fairly high inductance so a 50V system they will not be that fast. You will be spinning your ball screws and steppers way past the point of max power at 150 IPM if you want to do 3D work with small bits in aluminum. If you don't plan on using let say larger then 38" cutters most the time which for a router is about normal then going to 10mm pitch screws might be a good idea.
Steel will be quite hard to do. I would say anything larger then a .25" cutter would be too much for that type of mill. 2.5" tubes would help and wont cost much more. 16mm rails are fine too. If you can find them get med or high preload rails.
I would change a few things on the design but lots of things would work.
For small cutters at high speeds you will want to have almost zero backlash in all the moving parts so good AC adjustable end bearings and re packing the ball nuts for no backlash will need to be done.
I have a 4' double nut 2.5 TPI screw I could let go for really cheep. After I get my shop up and running I could do up the ends.