There's no way at all he has even $1000 in that rifle, let alone $1700, let alone started with a rifle he "paid too much for" and then dumped another $1700 into.
I don't recognize the optic, but the list of those for which I'd pay a few hundred bucks is incredibly short, and I'll guarantee you that's no Trijicon, EOTech, or Aimpoint. It doesn't even look like a Vortex, Burris, or Millet (some of the better budget level brands).
Personally, I think buying it off your brother at any price would be a mistake. You can't trust him, and that rifle is clearly a budget rifle tricked out with budget gear that he's trying to get quality rifle money for.