24-70mm will be ideal.
Flash or not depends on your preference, but personally, I'd recommend one.
That said, make sure you have the skills to shoot this properly.
Sorry, without your sample pictures or regular postings on CS with photos, I don't know what level of ability you have.
For myself, I only started shooting such events as a relative/friend and not the main photographer and only moved to covering it as the main photographer after I knew what it took and how to control the flash and camera settings in most situations.