I've been thinking about this question since I first saw it asked a week ago. I've been looking at a GH5 for myself too and I got to say it's a damn good camera. Like someone before me said, it's got a lot of bang for the buck. I'd put my money where my mouth is on this and get one for myself too if I could.
Though I have not used a c100, I have used a c300 and assisted for a c500. At my current day job we use an fs700. Personally, unless the c100 has significant changes from the other cameras in its series I would go with the c100 any day over the sf700. Maybe that's just me. The fs700 doesn't give you easy control of ISO for one thing, as in you can't dial it in without navigating multiple menus. I may be remembering the c300 incorrectly, but if it's like my 50d or 5DmkII you can dial in the ISO to the exact setting you want with relative ease. In the fs700 you can use a low, medium, or high gain instead that can be switched to ISO rating, but you can only switch between whatever 3 ISOs you choose for your low, medium, and high on the switch. The only way I've found to change this is going into the camera and setting a new ISO rating to one of your switch positions. That one difference between the two cameras is big enough for me that I would never consider buying an sf700 for myself. That isn't my only reason, but again, for me personally that reason alone is enough.
To elaborate just slightly on the overall differences, I haven't tested their images side by side at all in any compacity, but the c300 and c500 (and therefore I assume the c100) give me significantly easier access to the controls I want and need and the fs700 is more like a journalist's camera or something because it makes fine tuning of the settings difficult in favor of automating everything instead. I feel like the c300 and c500 let me shoot the images I want how I want and the fs700 is asking me to let it decide for me. And that could work for you, there's nothing wrong with that, it just isn't for me.