With 5mm pitch screws you'd be looking at increasing the speed of the screw to get a higher linear speed from a given motor rpm, though I doubt you'd be too worried about that on a machine of the size you're looking at. If you want to cut aluminium then keeping to a 1:1 ratio for more torque but less speed would probably be the advice of the experts.

Direct drive is simpler and a bit cheaper but makes the overall machine length a good 200mm or more longer than the screw. Belt drive allows you to change the drive ratio as described above and to tuck the motor under the frame for a shorter overall length. You'd probably want to go for a belt drive on the gantry anyway rather than have a sticking-out motor flailing about the place but it's not compulsory. I have direct drive on the long (X) axis and vertical (Z) axis motors and a belt on the gantry (Y) axis motor.

Kit