I assume, this map is meant to experiment with some features of cube2 like fog and models and to learn how to handle them?
Nice to see you figured out, how to make round looking stuff like some corners of those bricks. That one broken big pillar doesn't look to bad either.
Did you experiment with the fog commands already? There are many possibilities to alter range, colour etc.
I hardly use any models in my maps, so I do not know for sure, where to find the model you are looking for. Khorne released/converted a flora modelpack once though, use teh search! :)
If you want to improve your editing skills, my advice would be: hands off materials, models, heightmap, fog. I distracts you from the core of mapping: manipulating the cube, planning layouts, texturing and lighting.