There are special paints available at Lowes and Home Depot that are for painting over metal. You may need primer before you use normal paints. Always remove any rust before you start or the paint won't stick.
Yes. Use spray paint, to avoid brush marks. Start with a good primer made for application on metal. When it dries, spray two coats of the desired color over it.
YES,there is no problem in painting on metal. before painting you should make sure that the metal is not rusted. If so clean it thoroughly. While painting make sure that that the paint is evenly coated and no part is left out.
It is perfectly fine to paint metal, but you will need a different kind of paint that is designed to stick to metal. Regular interior paint will just come off.