Whiteboxing and 3D Assets

It's really up to you how you create the assets, but I'd suggest looking at this first:

Assets Creation Guide

And also have a look here:

All 2D/3D Tutorials

and here:

Asset Creation

I mean using all the external tools for terrain etc. is cool and you'll get some engine-independent skills, but it's only good as long as you can easily import whatever you've created into the engine. Also I know whiteboxing can be done in CE Editor. I'm not sure about the details but here's an article you may find relevant:

UDK vs CryEngine 3 SDK: Whiteboxing