If you have a more powerful computer (better than a core 2 duo laptop anyway...), you might want to look into the Unreal Engine. (It's freely available, and you don't have to pay anything to Epic Games unless you start selling a finished version.)
It has tools to import 3D models in a variety of formats (like it can import from Blender and Maya, probably photoshop and GIMP too. But you'll want to look into that yourself to be certain what it can import from.)
It has a built in AI system so you can easily set enemy AI/actions (though if you are familiar with C++ you can code your own custom set ups). And I'd imagine there are resource sites which can give you custom AI parts along with other resources.
And the level/world builder is fairly easy to use. (I mean I had a pretty basic world set up in about 30-40 minutes of just tinkering around. Anyone with more skill could create pretty good looking worlds without too much fuss since the hardest part is actually creating the 3D elements themselves.)
Otherwise a free game maker is probably your best bet.