I would recommend Vue. More powerful, and more compatible with Poser. If you can afford Vue Infinite, it's worth it for the "ecosystem" technology.

If you want to stick with Poser, RuntimeDNA.com has the best vegetation and rocks. Many are inexpensive; there are even some freebies.

The Microcosm might be enough for your needs. There's a version included in Poser 6. You can also buy it at RDNA. There's tons of textures for it there, too, free and for sale.