If you're lucky, it could just be the voltage regulator on the alternator. In my E90, the voltage regulator brushes wore down to the point of failure while driving highway speeds. It's typical for the drop in voltage to send a wild amount of warning errors lighting up the gauge cluster when this happens.

If the VR did fail, part of the alternator, that would explain the battery running out of power as well. The car will run off the battery until it's exhausted.

I could be totally wrong and maybe the used trans has a major issue. But I wanted to throw this out there.