there is a good chance you will be taking pictures in a tight space so think about the 17-40 or 16-35. Make sure you have a flash to bounce off the ceiling - and use windows for the catchlight. A dreamy gaze out of a windows is a classic pose that always works
Yep, the most important thing to get would be a flash, as that will do more for you than any lens upgrade. The 430EX will be fine, especially indoors...since you won't be triggering multiple flashes or anything.
Beyond that, it'll depend on the space. In a normal sized room, I found the 50mm to be pretty tight (on APS-C, so that's 80mm on your 5D), useful for only limited shots. It was great for getting tight shots of just the bride, but, that's only a small part of the preparation. You also want shots of the bridesmaids helping, etc, and 50mm was too tight for that
I used my 11-16 and 28mm lenses more (so for you, a 16-35 or even the 24mm primes perhaps). I think a 24mm and 50mm would probably be solid, along with a flash, and then if you need more reach for a specific shot, you use your 24-105L.