In my opinion it is far too large and weak for cutting aluminum on regular basis.

My machine is build of 45x90 but even that is not enough if I'd want some efficiency. Yes, slowly it can do it, but not with speed. I would not even think about building it with 40x40, not even half that size. Also, my machine is fixed gantry type, so the table moves in Y direction and the Z moves on the X beam, which is making it more rigid than a moving gantry type, like the one you plan for. The cutting area I have is 450x250, so even that is much smaller than yours. On the other hand, 100mm Z clearance is maybe a minimum, so that in my opinion should be increased. My version one had only 70mm, like most small machines, but I realized it was not enough so when I started with my upgrade I decided to approximately double that. I now have 130mm Z clearance, which I think is the minimum for any kind of serious metal works. Remember that you will need a very good wise as well, and those steal quite a bit from your Z clearance.

Even the spindle motor is too weak. The least you could do is to double that, but if I had some serious plans on working with metal I'd chose a low speed high torque spindle, not the cheap Chinese ones which run at 24k RPM. But OK, I don't know that motor, so perhaps that is exactly the right type for the work, though I think 0.7kW is not going to work.

I think you should revise your plans. Sorry.