We can be fairly sure that the early Buddhist traditions believed that there was such a man. They attributed many texts to him - those that make up the Sutta Pitaka of the Pali Canon. Beyond that, there is little evidence...
Given the nature of movements on the borders of the Brahmanic traditions around the start of Buddhism - it seems more likely than less that there was a charismatic founder. The earliest of the texts in those suttas (according to linguistic approaches to dating the Pali texts) seem to share a consensus regarding such a person.
That tells us little though. What can we guess at? Prince/King were probably an upgrading of local-clan-leader (the Sakyans, if we believe the tradition) / He was a successful charismatic leader; as was Mahavira founder of Jainism. He lived long enough for the Sangha, as a monastic order, to be established and flourish.... Even this requires some speculation...
What we do have is those texts you mention - in Pali - in Burma, Sri Lanka and Thailand - and some translations. What we know the early traditions believed is what is in them, and they are (some of them) interesting reading..
---
Dave W
http://dispirited.org---