So, there's something in the FAQ about releasing a patch for those, who only own Div 2, but still want to get some fixes and optimizations for the game engine.
Do we have to wait for that until October?

Yes. The patch will likely be created and tested with the final version of FoV, so any last minute fixes or changes made to the main game will be included, and the time and resources required for beta testing kept to a minimum (most publishers not being eager to do multiple rounds of testing if they do not have to).

I'd expect the patch about the same time as FoV. If your short on cash you can get the patch and play threw again then, hopefully, can purchase the expansion.

As I considered, I'll buy the expansion, but first I really want to try the original one out with the patch:)