So Mary was picked to be the wife of the Holy Spirit because her will matched His perfectly?
No - the unification of Mary's free will with God's is necessarily a condition of her immaculate conception, yet individually a choice of her own free will to remain united. She was set aside by God as far back as the Garden of Eden, upon the fall of Adam and Eve. I think it would be a little too trite to say that God chose her arbitarily - He certainly had to fulfill the prophets by uniting someone from the line of David with the virgin. But, like all of us who are saved through faith, she did not earn it per se. She was saved by Christ through the gift of the sacrifice that was yet to come and the gift of faith which He bestowed upon her.
To sum it up, Mary was not technically picked because her will matched God's perfectly - although it did, and by virtue of Her salvation from sin and her choosing not to betray that. She was chosen before she was conceived by Anna and Joachim. She was further saved upon conception from the stain of original sin. Mary essentially started out with the same state of being as Eve prior to the Fall. Both were perfectly united with God's will by definition (they were sinless) and by desire (they chose not to sin). Mary, however in contrast to Eve, was willed by the Father to trample the serpent (call it payback), and in order to do so, she could not be burdened with the sin of Eve. Like Eve, she was tempted throughout her life, but unlike Eve, she did not sin. When Gabriel came to Mary at the annunciation, he was very clear in announcing that she is "full of grace". This was necessarily the condition by which the Holy Spirit could become her spouse. This is the same condition by which you and I will enter heaven - full of grace - and be wedded to the Holy Spirit for eternity. And what's even more extraordinary is that God the Father STILL respected her free will to say "yes" or "no" to the angel. Until she said, "be it done unto me according to thy word", there was no 100% guarantee that the Messiah would come. It's astonishing to think about how much God respects our right to choose or reject Him.
So it could not have been just "anyone". It had to be Mary because it was God's will. But it was also Mary's will to do the will of God, which is, in a nutshell, everything every Christian should ultimately strive for in life. Mary was the perfect Christian. Not a deity, but a human being saved from original sin by her Son, and prepared to become the "ark" that would carry the presence of God until His birth.