I have a popular, lower priced, 3pt. 500# conical steel spreader that I bought new and have had it for probably 30 years. The steel drum is tempered and resists rusting......to this day, zip rust except where the stirrer wheel rotates in that little band and that rust is just very minor surface rust....you can see it if you look closely. Even the slinger cups aren't rusted out.A poly would be to my liking if this one ever started rusting, and probably would be the purchase.
Reason being is that upon job completion, I wash it good and then coat it good with used cooking oil and store it out of the weather, lying on it's side, where it can't have water or debris---like leaves, standing on it.
So the moral here is that the budget spreaders work just fine and the only problem I would see with a poly would be leaving it out in the sun and the sun having a chemical effect on the poly.