Environments in which to stage an animated scene I'm looking to shoot a short computer-based set of scenes, however I'm not sure what suitable (3D) engines/environments would be available to facilitate this.

I'm aware of the Source engine, in which I can place models and script the movement of characters - this engine also includes a number of existing character models, however my fear is that going down this route would result in the scenes having a "Clear Skies" feel about it.

So, what options should I be considering? 

(I am happy to create my own models from scratch using Blender or any other tools associated with the environment. I could also program the necessary scenes in software myself, however I'd rather not go down this route as it's likely to be much more time consuming than this project would already be!)

Also, the options do not need to be free or open source, as I am willing to purchase software if it looks worthwhile (but free is good too!) :)

One final note - while my main machine is a Mac, I also have windows an Linux based machines at my disposal, so recommendations can be for any of these platforms.

Thanks
Have you looked at Maya? 

Lovely showreel here. 

It's an excellent platform for animation, rendering and development.