Hi Paul,
it seems you came back at the right moment, there are plenty of toolsets at the moment that are gone free, so here are some suggestions:
– Cryengine 2 (available purchasing Crysis): imho is the best one for open environment and maybe one of the best graphically speaking; on the other hand documentation and support are not the best;
– Unreal Engine 3: recently they released the UDK for free. I haven’t used it so far, but I read that is great for closed environment and there a lot of tutorials and documentation for it (like 20hrs of video!);
– Unity 3D: they went free as well a while ago, and from what I’ve seen there are plenty of tools which it integrates with, really easy to use.
Hope this can help you!
Marco