I am a prosumer...so, let's start there. That being said, I have had the Samsung NX1 for over 6 months. I am able to use Samsung lenses, Nikon lenses and Sigma lenses on the camera. There are even mounts available for high end lenses, like PL mount. For the money (this is the key), I am not sure you can find a better image in 4k. Also, it takes very good stills.
Here are some videos I have made with the NX1:https://vimeo.com/timmyturntable
If you really want to stay around $700 and shoot 4k...which was my budget...you may want to also look at the Panasonic LX100. It has a really nice Leica lens on it and it tends to grade fairly easily. Just so you know what you are getting into...the video I shot below took up 23 GB of space before I cut it down to 2 minutes.