Depending on the layout of the house, instead of trying to retrofit ductwork, you might want to look into minisplit systems. Those come in many different configurations, some of which allow ducting to spaces you may not want a head in (like a bathroom), and, many of them are quite efficient. Second benefit, all of them are available as a heat pump, and can serve as a heat source, too, for not much more cost than an a/c only version. They would provide zoning, which may be harder to obtain with a ducted system, and other benefits as well. There has been a lot of discussion on this topic, so you might want to read a few of those threads, then come back for more specific questions to your situation.