It would probably be a not-insignificant amount of work to move that many buildings into Terragen. The ideal scenario would be if you could select them all and export them as a giant object with all textures intact and then hopefully Terragen could read that with all textures correctly assigned. This would ensure that the placement of objects relative to each other is preserved and doesn't have to be reproduced for 100s of buildings in Terragen, and would also cut down on the number of exports and imports you'd need to do. But depending on the number of unique textures and amount of geometry it may just be too much. And this also depends on the distribution of the buildings on the terrain being already correct in Modo. Is that the case?
If you're already planning to composite and have experience with Nuke, the easiest thing may be to just to render out the environment aspects from Terragen and output a bunch of render elements like atmosphere, depth, etc. Output a 360 environment map (spherical render) and render your buildings in Modo using that for lighting (you may get away with a somewhat lower resolution image since it's just for lighting purposes), and again output render elements like depth. Depending on what the ground area in/around the city looks like, and how you are planning to do any terrain elements, you may need to import or export aspects to/from Terragen or Modo to match that up for shadow casting, etc. Terragen can export terrain as heightfields or geometry, which you could use for correct shadows in Modo, for example.
- Oshyan