For this type of project, you want to think in terms of long-term decay. Only the strongest materials would survive, and even those would be heavily overgrown. Almost fully reclaimed by nature. Steel and iron would be heavily rusted, no shiny bits at all. Lots of plants growing over everything, so get out the green flock, paint some streaks on those buildings and create a million creeper vines going up and down the surfaces.
For some strong visuals, try a couple of my favorites for long-term-decay:
The Lost Future
Logan's Run (2nd half of the movie)
Enjoy.