If you build them out of mild steel, they will need to be coated with something decent. If they are supported well enough they should hold up.
I have built some stuff along these lines out of mild steel and had it work. But I had to make things thick and coat them.
These days, I feel like if I am going to go to that much work, I'd rather do what I can to prevent having to do it again, or fix cracks, or have it rust away etc.
Might be better in the long run to go stainless and be done with it... though I understand where you are coming from!