I used a Garmin Virb to do my G3X videos last year.
It took me a while to figure out the best way to get reliable audio. I found that using the headset patch cord that came with the Virb aviation bundle worked the best.
I also found that the audio would work best if I turned the Bluetooth OFF on both my phone and my Ipad while filming. (my phone was BT to the audio panel and the ipad was BT to the G3X.
I actually did not use all that much hard drive space. I did all my videos on my laptop that has a 500 GB hard drive. I still have plenty of space remaining today. It all depends on how much video you do.
However, I did think seriously about getting a new PC to do the video post processing - something along the line of an Alienware PC level of performance would have been nice.