I just did my first for a friend, and it is hard, goes by lightning quick, and there are no second chances. If you don't need or really want another lens, don't spend the $ on something else for their wedding - rest assured your services are worth a lot of money.
Myself, I rented a 5d and a 24-70 for the day, because I wanted to try the 5d, and the lens too, ha.
One body is not sufficient. Have two mounted and ready. If something locks up you need backup and you need it fast. I used the 24-70 on the 5d and my 70-200 on my 7d and switched easily between long and tight shots.
One other piece of advice that may help is to go over a checklist with the couple, to see what shots they want.
Things like, bride leaving house. Bride leaving limo. Rings. Cake. Flowers. Bride with family. Groom with family. Bride with bridesmaids. Groomsmen. BG with each set of parents, different combos of siblings, etc. Ceremony is hard bc you have to be in two or more places at once to get each version of vows and ring exchange. Bride with father/aisle escort. Groom shaking dad's hand. Bride hugging parents. All that stuff goes by so fast, and in my case, in rapidly changing light. If you can, get an idea from them, and the church or other officiator where you are and aren't allowed to go. Some don't want people on the alter, making your angles better pre-planned. Some don't want flash, or restrict it to non prayer or homily, etc.
I was all polite and turned off all my focus beeps, and didn't shoot (per request) during prayers and homily, and some dingleberry in the audience STOOD UP with a P&S during the prayer and took flashing, beeping shots. Argh. So, that's my story, lol.