@Jazz - thanks for the input. I can see the benefit of what you have suggested and I will look to incorporate the changes. I haven't purchased any hardware yet (thought it would be wise to get some feedback first!) so the move to 20mm rails all around is an easy one.

@ Robin - I undertsand the benefit of low overhangs, but I am looking to 3D machine parts up to about 70mm deep (a shallow trough form so I don't need a 70mm long cutter as long as the spindle body clears!). That combined with the 50mm tool height (and a little margin) gave me the 130mm height. I will still look at the deflection of the gantry at worst-case conditions to see if there are stiffness improvements to be had.