24–70mm f/2.8
$$$$Weddings are walk-around work. 24mm gets the aisle and the room; 70mm gets the toast and the first dance. f/2.8 is the stop that still works under mixed reception light without living at ISO 12800. A 24–70 f/4 plus a fast 50mm is the budget split.
What to look for: Constant f/2.8, reliable autofocus in dim light, a lock so it does not creep on a strap.