I have a Nikon. I know nothing about the newer models though, sorry. The best lens for low light is a prime lens, but there are some decent zoom lenses out there that go lower. I need a prime that is like 1.8 or 1.4 but I have other things on my money priority list right now.
One piece of advice would be to get a middle of the road vs. higher end camera body and take the money saved and put it toward the nicer lens. Unless you're going to go pro, you don't need the top of the line camera body for everyday (really beautiful) shots.