Well NPC’s is a totally different subject as compared to what would be considered a “hero” type model by design so for this purpose full body morphs would be a very effective use of resources but design wise your getting into making the next Assassin’s Creed territory that would require a rather extensive design document beginning at the top down.
As a starting point though to test theory as to practical application Daz Studio is an excellent try before you buy anything application as it already has key features to jump start the testing phase with out a lot of messing around remodeling but if you want to go with what you got morph targets are additive meaning you can change the shape of the head and it will just added the other targets on top as a +1.
The big question though is do your NPC’s need to act (ie dialogue) or just need to fill the back ground with using a few key expressions?