Let the fun begin !!! ???

[JAZZCNC]

i had intended to use 20 mm rails on the z axis but after reading the various posts of your good self I am now going to use 15mm rails and guides to suit so i think a redesign is in order.

No I wouldn't. There's very little price difference between 15 & 20 and the bearing size and overall height makes building the z axis easier regards clearence for ballscrew. With 20mm you still need to machine a recess for the ball screw but it doesn't need to be so deep. With 15mm the recess is too deep so you end up having spacer blocks and this makes achieving accurecy that much harder.
In the past I've made Z axis for people who use 25mm rails just to get around having recess or spacer blocks thou I wouldn't take that route personally.

I've got 180mm on my Z axis and I rarely use more than 100mm and often it's much less than 80mm and most of this is for tool clearence not actual cutting depth so don't go chasing length as you'll probably never use it.
The fact you will have adjustable bed means you'll find a balance between Z extension and strength for the type of work your doing. This is the beauty of the movable bed you are very flexible and can always find the best height to suit the job IF NEEDED.!!! . . . I highlight these words becasue in practice you'll rarely move the bed (Because it's a pain in the arse resurfacing) but instead find that balance I've spoken off.
In normal DIY wood working every day use your very unlikely to use tools with over 100mm cutting length so I find 100mm is more than enough on a adjustable bed machine as I can always bring the work to the cutter if needed or create clearence by lowering bed.