I'm soon to launch on a rewrite of a web site.  The site has many typical CMS characteristics, but also has HEAVY customization and integration with existing back end databases.

The one thing I absolutely can't afford is getting months into this project and finding that the customization/integration to DNN is just way too much trouble compared to what I could have just done from scratch.

So, what I'm looking for is a "getting started" or architecture overview kind of thing.  Something that describes the basics of what DNN is and does, and the basics of how you extend it.  For example, if extending DNN comes down to your module can do absolutely whatever it wants as long as it inherits from such-and-such, then I would know exactly where to focus my research to determine if I'm going to be able to do the kinds of things I want.