The churches that he wrote his letters to (church in Corinth, Ephesus, etc.) were results of his missionary journeys. Paul planted most of the churches that he wrote to, with a couple exceptions such as the church in Rome, which he had never been to when he wrote the Letter to the Romans.

As for how well known he was, most of the churches knew who he was, even if they hadn't met him. For example, the church in Rome knew who he was and wanted him to visit, which is why he wrote his letter to them. On top of that, the authorities, Jewish or Roman, targeted Paul as a large player in the Christian movement, causing him to spend quite some time during his ministry imprisoned.

It's fairly important to mention that Paul thought that his mission was to the Gentiles. So there were other leaders that focused primarily on preaching to the Jews. But as far as Gentile Christians went, Paul was their guy.

We actually just discussed this and the Pauline/Petrine controversy in my Bible as Literature class, but forgive me if I make an error, there was a lot of information to cover. Early Christians, if we can technically call them that, were in reality various factions, split mainly between the Pauline school of thought and Peter's, or Cephas'. Peter and the disciples of Jesus hated Paul, because he claimed to be an apostle despite never having seen Jesus. Moreover, Peter's group is still actually Jewish, and believed Jesus' death had no impact on the continuation of Jewish law. In fact, Gospel writers of the pro-Peter faction have Jesus state that he is there to fulfill the law, not destroy it. Paul and his followers, and later the Marcions believed the opposite, with Paul going so far as saying the if you are circumcised you might as well castrate yourself. Paul's lack of Jesus' life and birth narrative is evident in Mark, which is widely viewed as the earliest Gospel, and has a very narrow focus in terms of his miracles. Some other reason why those Pauline books are not completely Pauline is because later editors tried to harmonize these competing views and wished to erase any hints at disunity.

Did early Christians perceive him as a primary leader, or were there other leaders who got just as much attention but simply did not write as much?

while the other apostles were alive, Paul was witnessing to the gentiles while the others were more or less witnessing to the Jews. Peter was the head of the church and James was the head of the church in jerusalem. so in that sense paul was one of many

early "counter" Christian texts, like the gnostic gospel of thomas, show a focused attack on matthew and peter by name, but not really paul or the others, which might give an indication of who people in that area at that time saw as leading figures in the christian church

if by "early christians" you mean in the 4th or 5th century or something, then that's totally different. by then paul had become more influential

I know most of the original apostles were illiterate.

how do you "know" this, just out of curiousity?