cast iron parts are one thing - brush strokes hide well enough on the rough surface if you're careful.
But sheetmetal, especially wide areas, it's going to be obvious it was brushed.
If you're just trying to protect the metal - who cares, as long as you have some color on it. But if you want it to look nice, you need to spray.
(of course, spraying doesn't instantly equal nice paint job, but you can get good results even if it's you're first time, if you're careful)
If you're asking because you don't have a spray gun - even a low/mid price gun is going to give you good enough results.