Definitely a flash and tripod if you are going to be taking any pictures inside museums, churches, or basically anywhere indoors. However, most museums will not let you use your tripod or flashes. The flash pops will fade the art and the tripods would cause a pile up of people (according to the security guards). I even had my tripod taken away at a few places even though I wasn't using it
I took 90% of my pictures with the 12-24mm so that was a good pickup. Don't know where you are headed, but Rome is highly recommended!