From what I gather, the IBS ruins the in-body sound; so, you're only left with the option of off-camera sound. I've got a Zoom H1 and I plan on using it to the fullest with this camera. It's a cumbersome process to keep track of the audio and matching video files, but I find that if I do it immediately after sooting, it works.
So far, I've sold off a bunch of gear too... only my RX100 to go. Last month, I had bought a GX7 (before sending it back because the GX80/85 appeared) and just loved the combo of the GX7's Natural profile and an easy grading option of film convert. With the FZ300, Panasonic really gives you everything but a large sensor; however, I'd trade the microphone input for a large sensor, pocket-ability and the interchangeable lenses any day. The GX80 is going to be a great option for off-the-cuff style shots and possibly more serious projects too- a great allrounder. Some will say not an A camera, but in the right hands, it could be.