My 70D did pretty well (using a 15-85mm lens) at a friend's wedding reception. The light was far from ideal but the camera autofocused without too much trouble with just disco lights (it was between 7 and 11pm in December so no sunlight either). I had to ramp the ISO up to about 800/1600 but the video wasn't too noisy.
A fast lens may help but, depending on how much of the ballroom you want in focus, your depth of field might be less than you need. Alternatively, there are LED video lights you can buy relatively cheaply.
With the stills, is it not practical to use an external flash? Even with inexpensive third-party models, you can change the angle of many flashes so you can illuminate the subject without overpowering it/him/her with light.
Last edited by Rorschach
on Fri Feb 28, 2014 7:37 am, edited 1 time in total.
DSLRs: Canon EOS 7D Mark II, Canon EOS 70D
CSC: Canon EOS M3
Lenses: Canon EF-S 15-85mm f/3.5-5.6 IS USM, EF 50mm f/1.8 II, EF 100-400mm f/4.5-5.6L IS USM, Canon EF-M 18-55mm f/3.5-5.6 IS STM, Canon EF-M 22mm f/2.0 STM