Z Axis Design Help

[Angler58]

I'm a newbie to CNC and have been reading various posts on the MYCNC site and have now starting to think about my design. Initial thoughts on size are cutting area of 1000mm x 750, 80% wood, 20% non ferrous (aluminium, brass, copper), with an allowance on the X Axis to add a future 4th axis. I have no real overall space issues for the final build, although I am constrained a little by budget.
I want to start with the Z axis, as pointed out in various posts (and I think) this is the most important part (the part nearest to the cutting device) and its design drives the size of the X & Y axis.
I would like to hear other people's thoughts, inspiration, advice on.....

What design features contribute to the Z axis strength & rigidity and should be incorporated into its design in what order, (biggest effect first).

i.e (I've not put the following in any particular order ).

1. Spindle mount and Back plate thickness, Thicker = Stronger ?
2. Best Linear Rail Bearing placement on X, (both front, top & front, bottom & front, top & bottom)
3. X Axis screw placement (behind X, on top of X, in front of X & behind Z, below X), Is closer to the CofG better?
4. Include side bracing of the Z assembly (assuming Bearing or X axis screw placement is behind front face of X), to increase rigidity?
5. Spacing of Z axis bearings (Spindle mount), Further apart less flex?
6. Spacing of Z axis to X Axis bearings, Is wider more stable?
7. Anything else.
Thanks