No one knows the factory fan CFM, so it's kind of a crap shoot as to what is "right" to cool whatever setup you have.
Reality for MOST is that the electric fans are worth it only if you need to shut the fans off for water crossings. Technically electric fans will make AC work better, but that is ONLY if they pull more air than the stock clutch fan.
IMO stay away from the aftermarket stuff, and stay with OEM parts. The Windstars are probably the most popular swap, and I've yet to hear anyone say they don't cool as well as the stock clutch fan.