ac6000cw

I guess another consideration might be the level of video shooting experience - something like a GH3 (sticking with cameras inside the OPs stated budget) is a lot less likely to bite you with moire and aliasing problems if you get it a bit wrong. And mirrorless cameras have a viewfinder usable for video... Also if you have to do 'run and gun' documentary stuff, a camera and lenses that can do a decent job in full auto mode might be useful - pressing the 'iA' button followed by 'record' has got me footage of events that would have gone away if I'd had to spend time setting it up manually. (but these a just my thoughts as an amateur )

​....and you could buy two of those in almost new condition for £500, or one and a good used lens - the G6 is an underrated camera, it's got some features (like focus peaking) that the GH3 doesn't even have. Unfortunately the GH4 is well beyond the budget of the OP. In low light with the G6, I use a second-hand Tamron 17-50mm F2.8 (non-VR, Nikon fit) on a cheap Camdiox focal reducer to get the equivalent of a 12-35mm F2.0 zoom - total cost about £220, and it's easy to manually focus with the aid of focus peaking.