Minimum equipment for shooting an event in a dimly lit environment
A tough one. Certainly might be worth wording the client up that it's going to be dark. I'd try and do a test if possible to see exactly how dark it's going to be.

I don't think you're going to get a much faster lens than ƒ/1.4 without dropping a truckload of money. There's a Canon 24-70 ƒ/2.8, it's AU$1750 so it's not small change, but would probably be reasonable to rent if you want fast + zoom + autofocus.

I'd definitely look at a good monitor, if you're having to judge focus off the camera's screen you're going to struggle.

You could look at hiring a Sony α7, they have better low-light performance. You'd need Sony glass to go with it.

Would an on-camera light be possible? A bit more obtrusive, but you can at least do vox-pops or whatever and see what you're looking at.