and they will evolve into heptapods themselves
That is not part of the movie at any stage, so you can put that part right out of your mind.
The heptapod tell Louise that they will need help from humans in 3000 years.
This is the crux of the issue. The heptapods need humans in the future. However, humanity is not able to be in a position to help, based on their current "fate" (for lack of a better word for the future).
It's not just about seeing the future, it's about understanding the cyclic nature of the universe. This gives the heptapods (and Louise) a very detailed understanding of cause and effect, before the events actually take place. Effectively, they are able to understand what will happen in the future based on actions they take in the near future.
The heptapods therefore realize that if they pass their teachings on to the humans and give them another 3000 years to process it and evolve their civilization, that this will cause the humans to be in a position to help the heptapods when the time comes, 3000 years from now.
The movie suggests that only she has this gift
Because she's the first to figure it out. Not because she's unique. Her skills/character allow her to understand it before anyone else, but there's nothing preventing other humans from also learning this.
In short, the heptapods did not require Louise specifically, though their understanding of the future implies they did know they'd meet her and how they'd need to teach it to her. They presumably tailored their information so that Louise would understand it, and then pass it on to the other humans.
Is the implication that every human can develop the gift by reading the book
Louise's foresight came from understanding the language, so the reasonable inference is that if the book teaches the language, it inherently teaches the foresight as well.
It's not specifically proven that (a) literally any human can learn it (maybe some people just don't get it) and (b) that Louise's book actually teaches the language, as opposed to merely talking about her experiences in discovering the language.
However, it's heavily implied that the language/foresight is not the only thing humans need in order to be ready in 3000 years. The heptapods specifically distribute their information across the largest and most powerful nations of the world, in a way that the only way to unlock the secrets is to cooperate.
That cooperation will lead towards peace (as shown in Louise's future vision of meeting the ambassador), which is highly likely to lead to humanity making technological advancements that aren't focused on destruction and warfare.
Having an understanding of future consequences also would get people to reconsider going to war, as they'd much better understand how violence begets violence and how it's never going to be a quick and easy victory, and humanity will lose a lot in the process.
