Unless they are hanging white sheets from the ceiling, you are not going to get great shots with your flash. That's because you won't be able to bounce it off the barn walls or ceiling. That is the preferred method for using a flash at an event venue, and that's because without that you have to shoot the flash directly at the subject creating a very harsh lighting situation. I would definitely not buy a lens specifically for this event, and I would instead rent something like a 50 millimeter 1.4 or a 70-200 millimeter 2.8. Those are both staple favourites amongst a lot of wedding photographers.
But also don't be stressed out about all of this information flying at you. The person who asked you to shoot these images is going to be really happy with them as long as they can actually see what is going on in the photos. Another tip is to find out what's formal family shots she wants to get in advance. Have a list with everybody's name on it grouped with who she wants in what photos. I.E. bride and groom, bride and groom with brides parents, bride and groom with grooms parents, etc.