Yeah, I'm pretty worried to be honest that all this is a scam... and slightly worried for your wellbeing, and safety.
I'd wonder why they were just advertising in a paper, and how, if its a top business for one of the richest people in the world, how they'd accept you just like that, and call you for an interview only an hour after. You'd expect them to have tonnes of candidates.
$500 a month, for how many hours?
That said, I hope it is what it is, and if it is, you have landed a pretty good job.