Large aluminium profile build

[routercnc]

[*]single large profile for the X gantry beam like it is now, or rather break it up into an L shape as i see is popular?

Better stiffness in vertical and fore/aft directions, plus better torsional stiffness than if you just used one of the single profiles used for the | or the _ part of the L
To get these benefits they need to be joined to each other along the edges where they meet.

[*]iv’e seen some threads in which the Z axis was designed in a way that the bearing carriages are stationary and the rails move with the spindle - could anyone point clear this up or point me to the right thread?

With the bearing carriages stationary on the Y axis and the rails on Z you always have a support nearest to the tool, with minimum offset and therefore it's stiffer. Consider 3 scenarios of cutting height:
If you are cutting at full Z extension then both methods are the ~same (low stiffness).
If you are cutting at mid extension then rails on Z is much stiffer - offset from bearing to tool is lower.
If you are cutting with minimal extension then rails on Z is very much stiffer - offset is very small.

Sometime rails on Y axis is called the 'tuning fork' arrangement. Draw it out and you will see why, and you will see the difference at the different cutting extensions.