The 5D mark II for £440 is an absolute steal as you're getting a really good full frame stills camera, solid body and decent enough video quality. For me the most important factors In documentary shooting you want in a video camera is: 1) weather resistant (depends where you shoot) 2) good battery life (need to be filming all the time) 3) good in low light (can't control conditions) 4) small file sizes that are quick to import and edit 5) good audio connections, ideally XLR inputs but you can always attach something like a tascam dr 70d I think for £500 you could get a used GH3 which is a solid, weather sealed camera with good battery life for £400 and then spend the rest of your budget on a good lens like a voigtlander which will enable you to shoot in low light and with a shallow depth of field. That said i've seen the gh4 going for £800 and that gives you 4k which is really useful for interviews and re-framing shots.