What made Saul so special before his conversion that Jesus singled him out?

I think that is pretty easy. The Bible says about David that he was a man after God's own heart, and Paul in the New Testament was pretty much the same way. He was honest, he was devoted, he was not after fame and fortune, and God knew he had a good character. He was just headed in the wrong direction; he had not had the right education, the right information to set him on the right path. But once he was put on the right path, there was no stopping him. All God had to do was educate Paul, and in three days that was all it took, because Paul already knew what all the questions were, he just did not know the answers. Once he had the answers, that it was about Jesus Christ predicted in the Old Testament, he said let's go. That is the kind of man God wanted, because building the church in the midst of a field of demons is no easy task. You have to have a man with a heart that is hard and soft at the same time: a hard heart you cannot change, and a soft heart out of love for people and love of God. It is hard to find that kind of man, but God found it in Paul. You could beat him, torture him, insult him, take away his food and sleep, put him in prison, and he would not change; he was the same man going in as coming out. That is why God singled him out.