The 'long and strong' you speak of tells me you're planning on supporting the weight of the whole thing solely on the movement rails.
Wrong approach if weight is an issue. Build a sturdy support track, trolley, rollers, whatever method suits the application so that your motor only has to do work in the horizontal vectors and not constantly grind away at a load on threaded rails. When length gets long, you can start considering belts instead of lengths of threaded rod.