My niece has this 1200 grit whetstone and I just bought her the Arashiyama 6000 stone.
Unless I'm mistaken, I don't think she needs a Nagura but I do think she needs a flattening stone so is there a best flattening stone to get her?
A 400-grit Atoma diamond plate would be my personal preference. That said, a sheet of cloth-backed silicon carbide 220 grit sand-paper fastened to a flat surface, such as a piece of plate glass might be a cheaper solution. You could also use man-made laminates such as MDF for the "backing", but you have to avoid water absorption into the surface of the substrate.
The amount of time she'll spend flattening is probably not significant if she is just a home cook and has good knives. Get her a King 300, she will eventually need it, and she can reserve the bottom side for flattening. I think it will serve that purpose for her well enough for years.