Agreed, having the CAD files is best, but since that pre-supposes having the same CAD program available, STEP should be included as well. STL can work in a pinch, but if you have STEP, you can convert to STL easily, but not the other way around.
Often, even re-producing the part will prefer a STEP file at least. Odds are, the original was designed for injection molding and will do better as a 3D print with some modifications that don't affect function but help printability.
I use FreeCAD as the base here both because I use it regularly and like it and because anybody can obviously afford FreeCAD. Solidworks, not so much.