If you're in no hurry, just wait for another BRS group buy. We seem to do them about every 6 weeks.
I also read that your built in overflow confused you. The black wall is the overflow portion of the system. When in use, that won't be full of water, but water comes over the top from the main part of the tank to go down to your drain at the bottom of the tank and then to your sump beneath. As far as when you test to make sure you have no leaks, I'd try to get a PVC
plug to verify that part is water tight.