Re: Painting Alum Siding-
If your siding is extremely oxidized, I would power wash it until no further oxidation can be rubbed off. Siding in better shape can be merely painted with a quality acrylic paint without a primer. However, in this instance I would prime with an acrylic exterior primer, especially if any bare aluminum is showing. My bias is to brushing rather than spraying. Brushing aids in bonding by physically rubbing the paint into the surface. I would disagree with the statement that spraying is more likely to bridge crevices than brushing. In fact, my experience is the opposite.
As to what brand of paint is used: be advised that all the major manufacturers make multiple lines of paint from low end to premium. Ask and consult with your local paint dealer as to what line of paint the contractor intends to use. Generally, in this instance you want to use a 100% acrylic house paint.
A note on spraying: paint does not normally need to be thinned to spray if the proper equipment is being used. A powerful enough unit will support a large enough tip to spray the paint correctly.
A thought on priming: I know of no situation where priming is wrong, however, it is sometimes overkill. In your situation it is not overkill. Priming is done for adhesion and color change. In fact, if you are not dramatically changing the color, I question why 2 coats of paint are being suggested?