Hi all and thanks for your comments.

I suspected as much that the Z axis was the weak link.
I have had quite a few variations of the design that whas more stiff, but got to concerned about weight and scrapped the ideas. Then I started to think about Aluminium instead...

When it comes to the motors, I choose the closed loop steppers mainly to prevent racking on the X axis. (since im a beginner at this, its one of the main concerns)... It was not so much info to find on the net, but found some people that was very satisfied with this type of motors. I negotiated quite a good price for the set of motors and drivers, so I decided to just go for it. I understand that this still is just a stepper, but hope it will work good on this type of machine.

A few more questions as im not so familiar with aluminium
-Do I have to use Aluminium Tooling plate, or would 20mm 5083 Aluminium be OK? (Plate used in ship building industry)
-Does using Alu plate still require additional bracing of the Z axis, or is this much stiffer in it self?

I will have to work a bit more on the design of the Z axis, and I guess finally make a descision on Steel or Alu.