You can use a 70D, and it does autofocus rather well while filming. But a dedicated video camera is much quicker and easier to use, more flexible and generally much easier to carry in a pocket or bag.
My daughter shoots and edits video professionally and uses a 60D. But she is usually shooting interviews or other relatively static subjects, or actors that follow predetermined and practiced movements, and relies entirely on manual focus with Magic Lantern to provide focus peaking cues on the LCD and a Hoodman-style LCD loupe, and either off a tripod or shoulder support. Occasionally she shoots weddings and other events, and can manual focus pretty reliably after years of practice. While she likes the quality that comes out of the DSLR, even she is now looking at a dedicated HD camcorder. It's just too much effort with too little reward to shoot DSLR unless you're doing high-paid commercial productions, shooting 4K raw video on a full-frame sensor.
I'd do some research on full HD video cameras that can do 1080P at 24, 30 and 60fps with a mic input jack (eventually you'll want an external shotgun mic for better sound quality) in the $300-500 range from Panasonic, Canon and Sony. A manual focus ring would also be nice but that will increase your cost to over $800.