About the Z-axis... It is foolish to do anything other than what Jazz has suggested, namely putting the rails on the front plate. With the rails on the front plate the distance from the tool to the support (i.e. the bearings) is some constant (overhang of spindle plus length of tool) plus how far down the Z-axis is. So the minimum is the constant, and the maximum is the travel plus that constant. Whereas with rails the other way round the overhang is always the same constant plus the travel - i.e. it's always at the worst case scenario for the other orientation!

I don't really see the point of going above 20mm on the Z-axis, the gain is absolutely minuscule compared to the added cost (especially when you're using just 16mm round rails) as I'm sure you'll have found from routercnc's spreadsheet. I used 20mm for this reason.

Aluminium warehouse sell plate cut to size which is a good choice, as unlike the flat bar, it should be flat! It's a little more expensive.

http://www.aluminiumwarehouse.co.uk/cutting_calc.php

However, I urge you to consider not having gantry sides and using the style, like mine and Jazz's bigger machine where the cutter operates below the level of the X-axis rails. That eliminates the flexing of the gantry sides, as there aren't any, and puts the X-axis ballnut(s) in a much more optimal location in terms of the forces they receive.

Y-axis of 750mm sounds a bit much for just one ballscrew on X, especially considering you want to cut aluminium.