Paul was able to “translate” the eastern-thinking and felling of the Jewish people into the western thought process and logic of the Greeks. Thus, this new “way” became more “logical”, more easily accepted by the Western world. It has been said that if you ask a person of the Western world about something, you would ask “What do you think about that.” But to an eastern person you would ask “How do you feel about that.” Paul made this transition possible.
I think God chose Saul to convert the Gentiles. An odd choice since Saul was a very strong pro Jewish and not a supporter of the Gentiles.
I think the reason Paul was chosen was Christ knew he wanted to do the right thing even when he wasn’t. Also he had been trained in the scriptures which was a big plus. Add in that he was apparently very hard working and intelligent.
Paul even corrected the other great apostle, Peter about converting the Gentiles and Peter realized that Paul was right.