This is really a very interesting topic... and one I have considered myself...
I could offer the clean and clear cut, common answer that God wouldn't allow a man to take the position of Judas if it was not his place to take... And since Paul became the spokesman for the New Testament, then all is as God wanted.
... But this is a debate forum, so I will be honest with some of my doubts and concerns...
I don't necessarily believe things are clear cut in this issue. Since I believe God does not negate humanity's free will, I sometimes suspect that the Roman influence on early Christianity and the Biblical Canon was somewhat biased on the side of Paul... and that the men who decided how Christianity should organize inflicted their bias upon the following generations.
If you look at the early church, you see where Peter and Paul have their schism (James being on the sidelines)... but you only get Paul's side of the story... People conjecture as to Peter's motives, but the Bible isn't clear on that. So all we know is that Paul called Peter out about separating himself when other Jews arrived... Paul alludes to what he feels Peter's motives are, but without the inclusion of Peter's side of the story, we can't know for sure...
I have the impression that some of Jesus' Disciples' letters were left out of the canon, because they still followed their Jewish heritage, and some people took that to mean they were imposing their customs into the faith... Which may not be entirely wrong... but I just can't help but wonder about the other side of the story when I read passages such as Galatians 2.
I honestly think this is a complicated question, because I see the need for an Apostle who is not of Jewish origin, since Christianity is meant for all people. I do not see anything wrong with the teachings of Paul, and I think that he was indeed Divinely inspired. I still wonder at him having more face-time in the Bible than Jesus though. We know more of Paul's exploits than we do of Jesus'.
I just wish we were given more witness of men who walked physically with Jesus. I think the men Jesus personally chose to pass on his first hand accounts were probably better candidates than a man chosen by lots, or a man venerated by Empirical Christians... but this just me, and I admit I am fallible.