It's 1/35th scale. We've actually discussed this a couple of times here in previous threads. Yup, it's been announced forever, but now it's coming. Very cool. As far as I know, it's all plastic. I posted a link to some pictures of the master for the kit, and it seemed to be all plastic. (Don't know about the gun barrel, though.)
Anyway, I'm glad I held off on the Dragon one. This one has the transport cars and it's probably more detailed, at least if the K5 comparison holds true to the Morsers too.
From what I recall, both kinds of Morser will be buildable from one kit. Oh, and I think the carrier is a separate kit from the vehicle itself, which if true means you can use the Trumpeter carrier along with the Dragon gun. I don't think the gun would mind, particularly if the carriers are not portrayed attached to the vehicle (this might actually explain the delay -- mebbe Trumpeter wanted to be sure its carriers were completely compatible with the Dragon gun).
Don't quote me on any of this though!