-DBP - well you'll have to buy it.
-World Creator/Editor/Modeler - All you really need is models, from them there are lots of ways you can go, including making your worlds in DBP. Some things you might want to look at are Cartography Shop and 3D World Studio
-Models - There are lots of free models around, and lots of free modelers too, Wings3D for example
-Physics Engine - DarkPhysics seems to be the new black recently, but it costs money. Another one is Newton, which is very popular, but hard to use at first. You can make your own if you're experienced enough, which is what I did
-Anything Else? - Maybe some good texturing tools. Photoshop is quite god for textures.