Staircase repair: contractor or engineer?

The stairway in our building is a real mess; it needed some repair when we moved in 8 years ago, so you can imagine how bad it is now. The building has 4 units; 2 owner-occupied and 2 rentals. The steps are very squeaky and most all of the treads give under your foot (and are very noisy); the banisters are very wobbly. The stairs generally tilt inwards, towards the middle of the building. However, I am not seeing any signs of the stairs pulling away from the wall…yet. I had one contractor come in and look at it who originally told me it was just settling, but upon seeing the plaster cracks on the top floor told me I was dealing with a structural issue. I’m not sure what my next step should be. It seems to me that the treads could probably stand to be replaced, and obviously some plastering work could be done, but now I am concerned about a much bigger issue at play. Should I hire a structural engineer to survey the building at the cost of $500? Or get a second contractor opinion? Does anyone else have any wobbly staircase suggestions?