i would consider using aluminum first, then SS as a second option.
1" square aluminum is light and strong. SS would also be good and strong, but not as light. and high $
they both have the added advantage of being easily finished with or without paint. the SS would be easier to weld if you are a bit rusty on your TIG skills.
whatever you chose to use i would burn up a bit of scrap before embarking on the project, even if it means buying a few extra feet just to practice on.
FWIW; i found 1" aluminum easy to weld on and very strong when i made my welding cart.