Before the Annunciation by St. Gabriel, did Mary desire to be the Mother of the prophesied Messiah?
St. Alphonsus of Liguori argues in Glories of Mary p. 484 that she valued her virginity more than being a mother, but she valued doing God's will above all:
An author* says, that so much did the Blessed Virgin love this virtue [of virginity], that, to preserve it, she would have been willing to have renounced even the Dignity of Mother of God. This we may conclude from her answer to the archangel [St. Gabriel], "How shall this be done, because I know not man?" [Lk. 1:34] and from the words she afterwards added, "Be it done to me according to thy word," signifying that she gave her consent on the condition that as the angel had assured her, she should become a Mother only by the overshadowing of the Holy Ghost.
*Thought to be "the author of Oratio in natalem Christi (possibly Gregory of Nyssa); PG, 46, 1140", according to Philip J. Donnelly, S.J., S.T.D., on p. 241n59 of his "The Perpetual Virginity of the Mother of God" in Carol, Juniper B., O.F.M.'s Mariology (vol. 2)
Philip J. Donnelly, S.J., argues against this view (ibid. p. 241):
This, however, by no means implies, as some ancient Greek ecclesiastical writers asserted,59 that Mary set her virginity absolutely above the incomparable privilege of her divine maternity, and that she equivalently laid down to God the following condition: "I shall become the Mother of God, only if I may remain a virgin." Such a view is not to Mary's honor; it is in reality a denigration of her sanctity; it minimizes the dignity of matrimony as a divinely instituted and holy manner of life; it leaves out of consideration the supreme dominion of God, who could command any human being to make use of matrimony without any loss of sanctity.60 Rather, Mary was prepared to submit herself completely to the manifest will of God, even to the use of matrimony; she merely wished to be certain that the renunciation of her resolution, made initially under the inspiration of grace would be conformed to God's will; this explains very probably why she used the present (ou gignosko, "since I do not know man" [Lk. 1:34]) rather than the future [tense].61
Appendix 4 of Scheeben's Mariology (vol. 1), pp. 249-252, argues that before St. Gabriel explained how she could be both a mother and a virgin, she thought he was asking her if she would be the mother "of an eminent earthly king", not the mother of God. Certainly, she knew the prophesy that the promised Messiah would be born of a virgin (Is. 7:14), so it's conceivable she didn't yet know he was asking her to be the Mother of God.
In any case, she valued doing God's will the most, and He wanted her to become His Mother.