"tell me all the things that can go wrong with buying a steel 30 foot sailboat"
Well' that could be a long list, I will mention a few of steel specific issues.
Let's start with the design - a steel boat under 40 ft will be too top heavy, you just can't put enough weight in the keel to compensate for the heavy hull construction. This would be a show stopper for me.
Construction - big question mark here as we don't know how it was built or by whom.
Poor Maintenance - Steel will rot if not cared for, electrolysis can cause the hull to rot completely through in no time at all. Surveyor with steel boat experience is required!
Rust - Rust rust rust rust rust rust rust rust rust rust and more rust! If you were cruising the world on a steel boat then you could stay ahead of the rust issue. But it is a big job.
But you may choose to have this experience. I would think an older fiberglass boat would make more sense. Lots of good selection for late 70s and early 80s boats in good condition in the 20k range.