A professional would eliminate as much of the rust as possible through their method of choice, whether it be sand blasting, sanding, etc.
A follow up with a good primer, epoxy is excellent. A good body filler or glaze would be next.
They are in it to make money as efficiently and effectively as possible. Lead is becoming a lost art. Time and health concerns are a couple of good reasons.
You want to hammer and dolly out any dents as much as possible to reduce the amount of filler to a minimum.
Classic rust by the headlights? Expect holes and welding. If no holes, feel very fortunate.
There are many good fillers on the market. As for light work, Icing is very popular and great to work with.(if working with any body filler can be described that way)