Do you need to have interchangeable lenses? There are some really good compact cameras out there with large sensors and good zoom ranges that let in a lot of light. I have an Olympus Stylus 1 camera that has a 28-300 f/2.8 (the smaller the f number the more light it lets in and allows you to use in lower light) lens that continues to blow my socks off with its quality. It also shoots at 12 frames/second.
Second, stop worrying about megapixels. Megapixel counts are more for marketing than anything else. 12 MP or above should suffice for 95 % of the people out there.
Third, if you decide on an SLR (pick one, they're all pretty good), I would suggest getting an upgraded lens with it (the kit lenses with most cameras...not so good).
As far as flash goes, most cameras have a built-in flash which is really good for fill-in flash to lighten shadows during the day, but not much else. You might want to try it out for a while to see if you think you might need one. Cameras today are amazing in low light situations.