Ok, there's a few things you need to look at. How are you going to get to all the bolts to fasten on the bearings to the gantry and Z axis?

Are you welding everything, if so how are you planning to take the inevitable distortion out if the frame, gantry and Z axis? Epoxy? Shims?

The Z axis with the back plate does nothing for stiffness, think of a cornflake packet with the top and bottom cut off, and only makes it harder to align everything as adds more complication.

What thickness steel are you using? There's no problem in using steel but it will need bracing to stop it bending and twisting.

On an all steel machine (if thats what yours is?) you'd be best connecting the steppers to the ballscrews with belts and pulleys, this will stop any resonance from feeding back to the stepper motor.

Can't comment in the plasma thing as no experience with them, I'm sure a dual machine could be built but as with anything that doesn't do just one thing, there will be compromises.