For 3D (I can only advise you on 3D usage);
Blitz Terrain is a must have, no point in using the other alternatives, far superior; and there is a free version.
Dynamix; uses the PhysX GPU or software controlled physics drivers; why re-invent the wheel by trying to create collision and character movement using hard code? Using the CPU? Use the ferrari. Also a free version is available.
Dark Shader if you are building something that needs to look AAA.
As mentioned above; Matrix1; why? Because fills in many grey areas in the vanilla DBPRO command libraries. Advance2D; fastest and most stable 2D drawing library.
Blender is extremely powerful; too powerful for beginners, it's not newbie friendly; it does not try to be. It's more than just a modelling program, its a mini operating system; yes, it has its own commandline, game engine and API system, which means it can do almost anything. You'd better look at my article about
how to export from it before starting out with it. You will not be able to use your models in DBPRO unless you enable the feature.
3D World Studio is a must have for 3D level development; unless you prefer FPS Creator; you would be able to load FPSC levels in DBPRO. Mapscape is a free alternative to 3D World Studio, it has a displacement map facility, essential for sub-terrains and rough edges; 3DWS does not have that. Mapscape does not have 3DWS's terrain tool which is a little more sophisticated for large terrains; and does not have 3DWS's model browser, to see how models look before you place them; and supports more formats and has a Render to file feature. As you use both editors, you will notice that each of them has one or two features you'd wish where in a single editor. I tend to use them both; neither are as good as Valve Hammer, but they are pretty close.
Just take one step at a time and you'll pick things up.