While some may be a little heavier duty than others, I've never seen one that I thought was total junk. They'll all work if properly maintained. I do think the ones with the poly hoppers will last longer because they won't be prone to rust.
I got one this summer from an employee's father, he just gave it to me because he wasn't sure it would still work since he had left it out in the weather for several years. He was the 2nd or 3rd owner.
It has the metal hopper and it had rust holes above the gates. Some JB weld fixed it right up. Its also dented and the gearbox is pretty sloppy but worked great putting out lime a couple weeks ago.
If I was paying for one used or new I'd try to find a poly hopper as you suggest.