Yea I wouldn't use 5d3 if you got a
50K budget , It a Still's camera that does good Video but it's
not a
production Camera no matter what trick we do to the software to get raw video.
I would rent a C300 markii , C500 or even a 1DC or better yet the new C200 . Even thou the 5D3 can do 4k or very close to it you can't trust it other then short project's ,
it on the bleeding edge where anything can happen e.g. from corrupt frames to early stops , to overheating (FYI Canon "C" series has cooling fans internally) etc. ....
I know there are some that had great success in filming short's and some feature length projects but there playing with fire
as any thing could happen and you loose a day's worth or filming , is it worth it ? you be the judge . I would say run the 5D3's as backup's as there are not 100% reliable yet (if ever) .
I use my 5D2 in short news stories & mini Doc. (no more 30min) and I always have some issue , yes I know you are talking about a 5d3 here and I have a 5D2 but the point is even thou I know
100% that I can record crop mode 2144x1076 24p continuously (10bit) without problem I still have at least 2 other cams recording . 1 for different angle and the other for backup on the same angle of the 5D2 , as I had early stops for no reason (maybe got hot no sure) but I was covered from the B-cams . What ever you do have a mind set that the 5D3 could always have problems and needs to be monitored by a experienced user at all times to avoid pit falls .
My 2 cents worth
