The SDK and the D'Amiga
Is the SDK the BETA of the next Amiga?
The Amiga SDK, is NOT the next Amiga or the beta of the next Amiga. This is a Software Developers Kit. This means it is meant for those who are developing software for the next Amiga. SDK's are common in the computer software industry. Even mighty Microsoft used a SDK when developing Windows95. The reason for a SDK, is so that when the new Operating System hits the streets, there is already software ready to run on it. Without a SDK, you would have to wait longer for software to be written for your new computer and OS.
What is the D'Amiga?
Just like the SDK is not the next Amiga, neither is the D'Amiga. The D'Amiga is a computer platform that is created specifically to run the SDK. The reason for the D'Amiga is so that Software Developers have a common set of hardware to develop on. With this common structure, if a software package later needs to be 'fixed' for a later standard, that 'fix' can be applied easily to the other packages developed with the SDK.
Do I need D'Amiga or the SDK?
Only if you are interested in developing software for the next Amiga. I do recommend though, if you are going to develop software with the SDK, you also adhere to the standards of the D'Amiga, for easier compatibility and code upgrading.

How much of the SDK will be the next Amiga?
I will admit, I don't know. I would suspect that it won't look, or act, like the finished Amiga at all.  I suspect that it isn't meant to ever 'blend' into a true NextGen Amiga. Just as the early Windows SDK's were not a 'beta' version of Windows95 (though beta versions came later).   The SDK is just a tool, to create software, and may or may not look or act like the completed product.  This can be compared to writing code in C++ or AREXX though it looks nothing like the AmigaOS, the finished product will run on the Amiga.