Why don't you start with a reference design for a commercially available energy measurement chip, e.g. from Analog Devices? There application notes also given valuable hints about useful measurement methods. You didn't tell, how you intend to process the input signals and determine power factor. Apart from the said dedicated measurement chips, you can also use general uP or analog circuits.
Regarding current measurement, if you are targetting to an AC measurement (I guess so), a Hall Effect sensor is not first choice in my opinion.