Hasta La Motion!

I have a question about camprojection with nuke. I have a 3D model of an city, now I exported every building as an obj seperately and projected on every a texture from an psd file, then put every projectnode seperately in a scene then a rendernode, then merged them everytime with an over together. My question now is how do I get the Z-Depth from the whole city? Is there another way to merge all the files in one scene and lay them one over another, or does every projection need their own rendernode?

cheers

stephan