Welcome to CR! I'd recommend one of the f/2.8 24-70s. Either the Canon v2 or the Tamron. After you unload some of your Nikon glass, you can pick up a 70-200. Either the 70-200 f/2.8L ll or the 70-200 f/4L IS. The 600EX RT is the best flash, but a used 580EXll is pretty much the same thing without the built in radio. A pair of Yongnuo 622C transceivers will get you effective off camera flash with focus assist for cheap. Around $70 on ebay. As for the grip, I have an OEM one for my 7D and it works perfectly, though I hardly ever use it.