Are you installing vinyl siding? Does the window molding have to be the same material as the siding? I'm confused. Here is my house with hardi plank and the window moldings are painted MDF. We will install shutters, but if we didn't, these moldings would be wide enough to stand alone, I think. This is an old picture so excuse that the paiting is not finished.

Exterior trim like yours is painted every day. As long as the trim aluminum is clean and dull, any quality exterior acrylic enamel will do well. I prefer Duration Exterior, since it's self priming. Be sure all caulking is done with acrylic latex caulk and has at least 4 hours to dry before painting.

Would much energy efficiency be gained by replacing 30 year old Anderson casement windows? My garage was converted into a bedroom by the previous owners and they're definitely better than the aluminum windows in the rest of the house, but the room still feels cold. But that could be because of the total surface area of the window space.

If the bullnoses are available, then use them. It makes for a more cohesive look without adding another element. You may save a bit going with the metal trim, but that will go out the window when you'll need the metal edge corners; and if those are not available how well will the installer make those mitre corners?