Being new to CNC and designing a machine is quite the task, The table size you are looking at is getting up there in size, so to do milling as well as plasma requires a really stiff machine, the plasma cutting does not exert too much in the way of side forces.
Another thing to consider with plasma is usually you cut up to around 100ipm so I would recommend servo's over steppers.
Also most industrial plasma gantry's have automatic height control as it is important to maintain constant arc voltage.
I have used Acroloop control and software they do both a gas/plasma package and a mill package. Also depending on your work duty cycle, you may need a water cooled cnc torch.
Al