1) I'm not entirely sure since it's author has since disappeared. It's possible that his webshop from where you could order it will still work and automatically give you a copy if you purchase it, or that he is around still and just chose to hide and as such may give you the latest version if you tell him you wish to buy it.
2) Most of them will, however I never got DarkDynamix or DarkClouds to work with it if I recall correctly.
3) Not as far as we know. Its author haven't been seen for close to two years so it is assumed it turned into vapor ware. However the latest RC version that you could get if you purchased it overall worked out well.
Once it became more and more evident that it wasn't going to be finished I set about making my own version as can be seen here
; however when asked about it TGC didn't think it was a good project to pursue since it broke the protection on the licensed third party dll's. It is possible that Mistrel (the DarkGDK 2.0 author) reached the same conclusion and thus ceased his project, however he did get the special "DarkGDK Developer" forum rank so you would think TGC should have been well aware of what he was doing. So it is all just speculation.
4) No, it is slightly slower than GDK 1. It will be faster than DBPro where you do things that aren't actually DBPro-related (such as heavy maths, loops and whatever else you might write in pure C++ using it).
So in conclusion, I wouldn't attempt to buy it if I were you. It is true that GDK 1 is plagued by bugs that have since been fixed in the DBPro version, as well of lacking in new features and plugin support related to that. However both the DBPro and DarkGDK source code is open source now and basically both completely based on the DarkSDK, which means you can essentially just pick the parts from the DBPro source you want and add / replace them in the DarkGDK library and rebuild it.
This is what WickedX has been doing here
As for adding plugin support to DarkGDK you can generally do that. What you want is to call a few specific exported functions in the dll's you wish to use; I can loop up the exact names and order to call these in if you like. Basically you call Preconstructor ( if it exists), Constructor (if it exists), PassCoreData which is used to give the plugin a reference to the sGlobstruct pointer. Before closing your program you call Predestructor and Destructor (again assuming they exist). After you've passed the core data it's just a matter of calling the exported functions from the dll from your GDK project.