10 people wide from 10 feet is easy with FF with 24mm and back up double that stand on a ladder and you can get 60 people.
It all depends when you a going to FF and if you intend to keep the crop body or sell it. As Neuro has said many time the 17-55 IS is about the best general purpose lens you can get for a crop body and would be a very wise investment if you are keeping that body.
Wedding photographers are on the move a lot and sometimes the convenience of a quality zoom allows you to get the shot, with minimal fuss. That's why the preferred kit is 24-70or105mm and a 70-200mm. Especially if you are learning like I am.
Good luck with your decisions.