Would 25mm square box with 3mm walls be any good for the legs and framework?
works perfectly well for me

Mine's bolted to the walls and floor - got some 50mm square plates cut when I got the top cut (2mm mild sheet) welded them to the legs and bolted through.
Much more bench than that seems overkill to me. Lathe bench is made the same, takes a 9" SouthBend OK.