Hi,
your choice will largely be affected by what you can find locally cheaply.
While steels can a have a wide range of strength properties they vary very little in stiffness. Thus a high strength high alloy steel may have a Youngs Modulus of 230GPa,
and a mild steel may be 205GPa.....so despite very widely differing strength properties they are remarkably close in terms of stiffness.
I would suggest a readiliy available medium tensile mild steel. This would allows you to cut, drill and otherwise process into the required shapes, have a moderately high thread strength
and will be nearly as stiff as some 'you-beaut' hardened and tempered high alloy steel, costing twice as much and a pain to work.
Craig