The problems that Elon will face if he indeed creates this political party are more than one, and all of them are huge to overcome, just because he has money. Trump has nothing to fear from this move. Plus, and most crucial, is that to be a US President, you have to be born in the US, and Elon isn't. So, from the start, we are talking about the total nothing.
Yeah, it is a huge task to start a political party in any case, no matter how many funds you possess. The attention may just be caught by Elon and some resources that he has, but the fact is that it takes time and people to establish some political power. And you are correct in the nest eligibility matter since he was not born in the US his candidacy cannot run in the position of the president so that puts a limitation on the things he can directly be involved in. I believe that this step could stir up the discussion a bit, yet I suppose that Trump is unlikely to be ousted by Elon in the near future. Politics is not all about money or fame only.