Another argument I have heard is that Paul's writings presuppose a historical Jesus. After all you cannot have a death and a resurrection without a prior life. I think there is some logic to this (although mythers maintain it need not have been an earthly life and death), but I don't see how it helps the case for a historical Jesus.
Paul had an experience on the road to Damascus that he believed to be an encounter with a resurrected Messiah. While this may presuppose that this Messiah had lived and been put to death, it does not presuppose that Paul had any historically reliable evidence for that life and death other than his experience. We could just as well say that Joseph Smith's vision of the Angel Moroni presupposes that a race of native Americans with Caucasian skin tones had lived hundreds of years before his time. So what?
Paul's first source of information about this resurrected Messiah would have been the people he was persecuting prior to his conversion. However, I think experience shows that religious zealots tend not to have reliable information about their victims' beliefs. Jews were believed by their persecutors to be sacrificing Christian babies for their rituals. The Romans thought that early Christians engaged in incest because they called each other "brother" and "sister." The Puritans thought their victims in Salem were witches.
I have often wondered how much of Paul's early information might have come from informants or torture victims who would have been happy to tell Paul any crazy story he wanted to hear. "No. No. I'm not a Christian, but my next door neighbor is and he told me that Jesus appeared to 500 of them at one time!" Would Paul have needed any evidence beyond his vision of the resurrected Jesus to convince him that all the stories he had heard were true?
Christian apologists claim that Paul could have obtained historical information about Jesus when he visited Peter and James in Jerusalem three years after his conversion. Gal. 1:18-24. However, right before he describes that visit, he says "I want you to know, brothers, that the gospel I preached is not something that man made up. I did not receive it from any man, nor was I taught it; rather, I received it by revelation from Jesus Christ." Gal. 1:11-12. Given Paul's superior education and his history of dealing harshly with people who disagreed with him, I cannot help but suspect that it was Paul who did most of the talking at that meeting. Paul was already enjoying success as an evangelist and Peter and James may have been more eager to get on his bandwagon than point out any errors in his historical understanding.
The apologists also point to Paul's later trip to Jerusalem as proof that he got information from the original apostles. "I went in response to a revelation and set before them the gospel that I preach among the Gentiles. But I did this privately to those who seemed to be leaders, for fear that I was running or had run my race in vain." Gal. 2:2. However, nothing in what Paul writes suggests that he was at all concerned that he had gotten anything wrong. Rather, he was concerned that the guys in Jerusalem were behind the false teachings that were screwing up his work. "This matter arose because some false brothers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves. We did not give in to them for a moment, so that the truth of the gospel might remain with you." Gal. 2:4-5. If the apostles in Jerusalem had disagreed with Paul, it would not have changed his message. "As for those who seemed to be important—whatever they were makes no difference to me; God does not judge by external appearance—those men added nothing to my message. On the contrary, they saw that I had been entrusted with the task of preaching the gospel to the Gentiles, just as Peter had been to the Jews." Gal. 2:6-7. Paul then goes on to describe a later meeting in Antioch where he accuses Peter of hypocrisy. Gal 2:11-21.
Everything that Paul writes indicates that he considers his knowledge and understanding of the gospel to be equal or superior to anyone's based on the revelation that he had received. Had he thought that those with whom he disagreed had been personally taught by Jesus for three years, it is hard to believe that he would not have grappled with the fact that others had sources of knowledge that were not available to him.