In blogging through the Summa, I'm going to start with the third part because:
1) it's about Jesus Christ and the sacraments, so it deals with material we're more familiar with
2) it's less heavily philosophical than the first part. To start at the beginning could lead to discouragement rather quickly!
3) while all of the Summa is profitable for spiritual growth, the third part is especially fruitful in that regard.
Question 1: The fittingness of the Incarnation
Article 1: Was it fitting for God to become incarnate?
Quick answer: Yes
Why? Thomas says that God's very nature is goodness. As Dionysius shows, goodness implies self-communication. So because of God's goodness, he has communicated himself to us in such a way that the second Person of the Trinity became a man. Thus, the Incarnation was completely fitting. God communicates himself out of sheer goodness, not driven by any necessity.
Thomas' sources: Note that in this article, Thomas quotes three of his most important theological sources.
1) St John of Damascus, who is very influential on Thomas' Christology. John lived in the 8th century (died in 749), the last of the Greek Fathers and known especially for his work On the Orthodox Faith.
2) St Augustine, who was also a huge influence on Thomas
3) Dionysius the Areopagite, also known as Pseudo-Dionysius. He wrote on the angelic hierarchy and on God, especially in The Divine Names. Because in the Middle Ages he was incorrectly thought to be the Dionysius converted by Paul at Athens, his work became very authoritative. He actually lived in the 6th century and was an unknown author who probably lived in Syria.
Thomas also quotes Scripture often. In fact, he equates theology with the study of the sacred page, the Bible, in the very first question of the whole Summa.
In his theology of the body, St John Paul II took up the principle that goodness communicates itself. He used it in explaining one of his key ideas: that as human beings we find happiness and fulfillment in making a gift of ourselves to others in love. This "freedom of the gift" was at work in the Incarnation. The more closely we are configured to Christ, the more we will live this and find happiness.