The purlin system will work well for this application. If using rafters you could extend the ridge and plates to carry the outside rafters, but I would prefer purlins. My point is the purlin to rafter connection, if you lodge the purling on top of the rafters the building will suffer no loss of strength, as show in the depiction.

Here is a video showing lodged purlins on three of the four bents, this building has no major over hang at the gable ends. By-pass purlins on bent three, bent one has tusk tenons. These have daps to hold the purling others I have done have no daps and are a great time saver.
http://www.youtube.com/watch?v=NLD8-CEmTLE