This is a long post and you probably can ignore me, but:
Although I do respect lots of people who use Bryce 5, I can only say this: You're gonna be limited by it. It has no NURBS, no polygonal modelling selection, no deformers, no (bla..bla..bla... you know the point
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Therefore, getting a low poly environment (which is simple enough) is, unfortunately, HELL. Bryce has a terrain generator, but it's not detailed and has no zeroedges function. Besides that, trees are hipoly yet are hampered by the textures (they don't have much normal mapping done)...
Another alternative is Vue 5 INFINITE, which is the real deal. This fall, I think a new interexchange application called Vue Fusion is gonna be out, which lets you work in a Vue-like interface, then later port the scene to 3DS MAX (my dreamlike tool), Maya (robust but hard), Cinema 4D (Easy, great, awsome but lacking in some features), and Lightwave (the only one in this area without NURBS but is surprisingly cheap and has a renderer quite good)
Really, to layout a scene in Bryce or Vue, (NOT in my opinion) isn't really "modelling" an environment- besides buildings that you make, everything else- and I mean EVERYTHING- is inbuilt. Go try some of the apps and import landscapes from Terragen- a free terrain generator, make your own models, and get the textures done- it may seem painstalking, but BELIEVE ME, it's totally worth it.
(BTW Vue has a quite noisy output when rendering images with procedural textures, and bryce can't handle terrains with lots of spikes)
Final word: If you want an image with a landscape, terragen. If you want plants and rocks, try Mojoworld. If you want them to be in-game, wait for Vue Fusion. But overall, it's the best to do everything from Square One. lol!