Sorry i meant to reply to you sooner
Heres a story we narrate in the following book finality of Propheood
Page 8 and 9
Basically Muslims believe that every nation or people had a Prophet sent to it. We believe in the ones in the Quran by name but we also know that there are some we do not know.
Our criteria would be that he call to the worship of one God.
Was Budda a Prophet? I don't know thats the answer there are signs that he had a powerful message because he broke down the caste barrier in his locality. If he was poor then people could've said he was poor and that why, but he was the son of the Raja so he was rich and he didn't need to.
I hope this answers your question