Based on the earliest historical records that still survive, we know very little. He founded a movement (he did not see himself as the founder of another religion, but as the "corrector" of the existing Abrahamic tradition), he was a trader before he became a religious preacher. He seems to have been a rather apocalyptic messenger (when he spoke about the Day of Judgement, one gets the impression that it was imminent).

Other than that? We know what the religious tradition tells us, but much of it from later on; biographies don't appear for at least a century, and they were compiled at first primarily to understand the context in which the verses of the Qur'an were authored. (At the time, history had a different purpose than it did today--it was meant to explain why things are the way they are, not to record things that happened in an accurate manner so that future generations can study them.)

Linguistically, the language of the Qur'an does suggest that it is authored by a single person (there's a study about this), and the discovery of the 'Birmingham Qur'an' manuscript a few years ago places the text to around the time he is said to have lived.

There are a lot of polemics back and forth, and tbqh it's a slippery slope. It makes no more sense to try to diagnose him medically based on the historical record that exists than it did for the US senator who claimed he detected higher brain function in Terry Schiavo by watching CNN.

Personally speaking (PhD in Middle Eastern history, although early Islam isn't a field I research), I tend toward Occam's razor: in this case, I think the likeliest explanation is that Muhammad was (or believed he was) who he said he was.

Pretty much since the advent of Islam there have been those who claim he was a power hungry madman bent on his own self-aggrandizement; as the motto of the state of Ohio says, "with God, all things are possible," but what I don't quite find satisfying about such explanations is they only address his rise to power from obscurity. They don't explain why he attracted so many people to his movement -- he wasn't the only person in Arabia claiming prophethood at the time, and none of the others who tried that particular tactic were even remotely successful.

This is, of course, all speculation on my part. However, if you want to understand the way that Muslims today think about him, the traditional narrative is the one that you would refer to.