How did God prepare Mary to be the Mother of Jesus and of the Church?

Catholics believe that Mary was born without sin (the Immaculate Conception), since Jesus would otherwise have inherited original sin from his mother. Against this, Luke 1:30 says the angel told Mary that she had found favour with God, implying that it was not pre-ordained (as the Immaculate Conception would imply) for Mary to be the mother of Jesus.