Don't post often but I felt the need to warn you:
Don't use a sanding sealer under polyurethane, the poly won't bond well and you are almost definitely going to have adhesion issues. Your top coats will likely start peeling in a matter of months, or sooner. You actually don;t need a sanding sealer, it has no function for what youre trying to do. If you feel the need to wash coat with something to lock in oils of the wood or something, you can do a washcoat of shellac and sand it flat, shellac will has no issues with a poly coat on top. Most likely its fine to go straight to the poly on bare wood, your first coat you can consider your sealer coat, sand it flat, and keep on building film with additional applications, with a bit of sanding in between coats to remove dust nibs, high spot, generally keep it level before a final sanding and buffing once all the coats are finished.
As for sanding, it makes no difference how high a grit you sand to, as long as there are no scratches visible. If you go to way too high of a grit, like to the point where youre polishing the wood, you may have adhesion issues with the finish. Once the film finish is on, there is no difference between wood sanded to 150, 180, 200, 240, whatever, so long as there are no scratches seen.