Get some framesaver (google it if you don't know what it is) for it if you do alot of wet riding or store your bike in the rain.
Many manufacturers put some sort of framesaver like coating on the inside of it before you get it but probably not ironhorse.
You will be fine. I have several steel bikes, one that is ten years old and the only place it rusts is where the paint has scraped off. Some light sanding and touch up paint is all it needs.
You shouldn't think of Aluminum as an upgrade from steel. Many of the finest bikes in world are made from steel.
Not yours, but many other ones.
that being said, if they said it was aluminum, and you want aluminum, they should let you switch the frame to an aluminum model.