Depending on what sort of room you're going to be in (ie- if it's a bit more cramped quarters), you might like a slightly wider lens than the 24-105 (I've loved my 16-35 2.8L so far- though it will distort people towards the outside of the frame). Other than that, you should be good with (primarily) the 5diii & 24-105. If you're in a larger room and you can get wide enough with the 70-200, though, it'd be a great lens to use.
With the 5diii, I wouldn't be afraid to go even higher ISO than the previously mentioned 3200. It does great, and provides perfectly usable images (IMO) up to 12,800 (though that's starting to push it if the shot gets underexposed). If you use the 7d for anything, you'd be pushing it a bit at 3200, though if it were me, I'd probably just have it around in case of catastrophe or if you for some reason needed the longer reach of the crop factor camera.
As far as diffusers, I've been using various versions of the Gary Fong Lightsphere for several years and think it does a great job, though I shoot primarily natural light (almost exclusively, unless I really just don't have a choice). The current collapsible one is much more convenient to pack & carry than the previous versions.
Good luck!
