If she is not going to do the quilting on vacation, and she is not going to use a fabric from the top for the backing, I would wait with it. But if she is going to use a fabric from the top, I would get the backing when you buy the fabric, otherwise it might not be available when you go to buy it later. Muslin is a good choice and can be purchased almost anywhere, anytime.
But I agree, decide what first (lap, queen, wall hanging...), then pattern, then fabric. Cut the big pieces of fabric down to manageable size that makes sense for the pattern (I try to avoid having to deal with yardage in kits)
Mike