I saw a documentary on Buddha that claimed that some British (I think) discovered some early archaeological traces of Buddha's historicity, but I haven't verified these discoveries myself. I confess that my interest for the history of Buddhism haven't been as great as the history of Christianity.
Anyway, historicity is interesting if Buddhism gave inspiration to Jesus or if Jesus was even based on Buddhism. Narrowing down what kind of evidence there is to place Buddha 500 bce is necessary to even begin to trace the line, of there is one. If you can place him there, there are 500-700 years between them in which the tale could have changed bit for bit, generation for generation, into what eventually became the gospel according to Mark.
I'm inclined to simple explanations. Mine (for both Jesus of Nazareth and Siddhattha Gotama) is something like this:
Somewhere around 30 CE and 500 BCE respectively, individuals named Jesus son of Joseph and Siddhattha Gotama respectively, walked around Galilee and Northern India respectively, saying things that impressed a significant number of people enough that they started passing on whatever they had said.
Later, lots of other things that other people had said, or that were just generally regarded as something worth remembering, got attributed to them. Since the biographies of both individuals were considered relevant to what they were saying, a lot of stuff that had happened to other people, or that was just plain ol' folklore, or even completely made up to illustrate a point, was ascribed to their respective lives.
By now, it's as good as impossible to say exactly what each of them said or did, and what's been attributed to them after the fact.
This means that it's entirely possible that ideas or events that happened to, or were originally attributed to, Siddhattha Gotama would have made it to Galilee by 30 CE, and have subsequently been either repeated by or ascribed to Jesus son of Joseph.
Does this mean that Siddhattha Gotama and Jesus son of Joseph were the same person? Not in any meaningful sense, from where I'm at. Does it really matter? Not all that much either, IMO. If an idea has worth, it doesn't matter where it originates. The Buddha may never have transmitted direct insight to Mahakasyapa in the Flower Sermon, but the
story of the flower sermon contains an important thought nevertheless. The Christ may never have risen from the dead, but the
story of the Resurrection remains significant nevertheless.