If your mostly cutting wood then this design will be fine but if your planning to cut aluminium often then I wouldn't use it.
For wood etc then the gantry height is fine and will be more than strong enough but with aluminium you need much more stiffness so I'd change the design completely.

Lee is correct about the Y/Z axis being a little wide for the size of machine.! 200mm is what I would use on machine this size.

Leave some room for overrun when placing limits because while they do stop the Control and software instantly inertia will push the axis and obviously the faster it's traveling the more it will over run. Asking how much is like asking how longs a piece for string.!!

Yes 10mm pitch on X & Y 5mm On Z. You won't need any ratio applied to the X & Y if you use decent drives and run them with good amount of volts. You'll easily it over 10mtr/min and you won't ever cut higher than that so don't waste time and money chasing speed.

Whats the frame and bed made from.?