As far as I know, the only evidence having claim over the fact about Ashoka's conversion to Buddhism is the Ashokavadana, a work of the Buddhists themselves. Nor is this fact verifiable by Ashoka's Edicts or Pillars.
Do we have any independent evidence (documentary or otherwise), about Ashoka's conversion to Buddhism? Maybe through the writings of any of the Greek Historians like Megasthenes or somebody?