I think you are on the right track. Basically you need to find a way to build the shape, then harden/reinforce it, and then the detail work and such.
Since it's a bit big I would say use foam of some sort to get the basic shape and bulk it up so it's not as heavy. The amount of clay you'd need to get that large of a shape would probably get a bit expensive. While paperclay and model magic are a bit light they're still not as light as foam.
Paperclay and model magic are basically both lightweight, air drying, clays. The main differences between them are that model magic is lighter and fluffier (it's easier to get finger prints in), but paperclay can be sanded. Personally, I prefer paperclay.
I like resin and fiberglass a lot, it'll make it nice and strong. But keep in mind that fiberglass cloth isn't really smooth, so your surface won't be smooth after you apply it.
As for the auto body filler, that's more personal preference. It's much harder and sturdier than paperclay but you can't apply it as precisely.
I made something semi-similar (it wasn't nearly as detailed) out of an old bicycle helmet and carved spray foam.