Do you now anybody in the body shop business? "Wax and grease remover" is the best wipe down product to use prior to painting. It would only take a few ounces to wipe down your parts, so any body shop would likely give you that tiny amount. But any good solvent like acetone or tolulene or MEK will do okay.

The etching primer is a very good idea. And if you want a good stable final finish, it is best to use paint that requires a hardener mixed with it. Most call it single stage, the kind that you mix 2 or 3 parts together and spray with a gun. Aerosol spray paint looks good, but is not nearly as durable.

Or you can consider powder coat. It is not very expensive for already prepped parts like yours and you can get most any level of gloss or flatness that you are trying to achieve. Although, I doubt that it is viable for your manifolds.