I too started with my X-axis rails mounted to a torsion box made from MDF/pine with a gap left either side to hide the 2 leadscrews. I'm still using the torsion box as the machine bed. Making the steel frame for mine made a huge difference. What size are your X-axis rails, they look like only SBR16 or 20mm?

Plate is weak for the gantry since when the machine is cutting parallel to X the cutting force will try and bend the gantry, and that's in the direction that's worst for a rectangular cross section. It's also the worst cross section to choose for twisting. Adding extrusion, or more plate perpendicular to it to the back would help, but you could try it and see and make sure you leave space to add it at a later date if required. What is the width of the plate, so I can calculate the deflection (roughly)?

I'd be inclined to add some triangular pieces to the aluminium angle to stop that flexing.

3nm is a good choice for the stepper motors. Lots of people, including me, use them. If you can afford 70 volt drivers (like MDS752) then you will get a higher feedrate than 50V (like MDS542), but it's more difficult/expensive to find/make a suitable power supply. The mass/inertia of the gantry is not a problem with these motors. Chip's and mine (and many others) using the same motors weigh a lot more.

Have you bought the ballscrews yet? If not then get 10mm pitch (RM1610) for X/Y to get a higher feedrate and use HTD timing pulleys (although 1:1 ratio is about right for those motors and 10mm pitch screws adding the pulleys still helps as it reduces resonance). 5mm pitch screws (RM1605) will work but it wont go anywhere near as fast, or more importantly the acceleration will be less and possibly vibrate more as the screw is spinning twice as fast.