3-axis moving gantry design CNC style 3D printer

Hi all,

I joined cnczone last year and wanted to make my own CNC milling machine. After lot of research I finally gave up because of space and sound constraints in my apartment. Then a few months ago I was making an iot temperature module but none of the enclosures I bought were good fit, plus they all require a lot of manual post-processing to make the correct holes and rectangles for connectors. So this drove me towards a new project, make my own 3D printer.

I researched a lot about 3d printers, looking at multiple designs(i3 prusa, mendel, delta, etc) but none of them use linear rails on all 3 axis. Looking at the differences and similarities between the two machines I realized that a 3D printer made using linear rails could easily be converted into a CNC machine. On top of that if I design a 3 axis moving gantry then I can have a machine with nearly 100% build volume which will take care of my space constraint.

Now my questions are :
1) I am considering a build volume of around 400mm(x)X400mm(y)X500mm(z). Will a moving gantry have sufficient strength in an all metal construction? What if I move down to 300mm(x)X300mm(y)X400mm(z)?
2) Aluminium is lighter but steel is stronger. For a moving gantry which will be preferable?
3) Based on what I have read I need 3 (ball screw + linear rail + support) for each axis. Something like this https://www.amazon.com/Happybuy-Ball...0483684&sr=8-7 . Other then this I need a base plate to put everything on, 1 support plate for y and 2 for z axis, another ball screw so that I can drive x axis from both sides and 4 motors(2x1y1z). I have not considered electronics/heatbed at all because I think I have sufficient clarity on that. Is their anything else required to complete a basic frame for 3D/CNC machine?
4) The bed on which all this is going to be placed. Should I use aluminium or steel to provide sufficient stability? What is the minimum depth I can use for bed? How do I get a bed with pre-drilled holes? I have a black&decker hand drill. Can I drill reliable holes using that?
5) Is their a 3 axis moving gantry design or BOM available to refer to? Can I just make paper drawings, buy stuff and wing it or do I need to make a CAM model before I jump in?

I should also point out that when I move to a bigger place with a garage, I plan to add a CNC module on this machine so that I can mill aluminium. That's why the machine needs to be sufficiently sturdy and precise.