At lower feeds which you'll run for steel and aluminium you'll be probably be ok but they will struggle with higher feeds or heavy cutting conditions.
Personally I'd be looking into small servo's or at least Nema 34 motors and high voltage drives. Even small 400W servos will be fine has they give linear torque so won't restrict machine potential.

The 3Nm motors and low voltage you plan will be very limiting to machine performance has speed increases.! They should handle lower feeds ok but seems a shame to build what sounds like is going to be a very well built and sturdy machine and not be able to exploit it's full potential.!! . . . . That said they will get you started but think you'll probably quickly want to upgrade.