About the pronoun “one”

Question

and the choice of what to include and what not to include was necessarily a personal one

Could it be said: "...was necessarily personal"? Which is the "one" to substitute, "choice"?

Answer 1

In the sentence you wrote, one is used instead of choice.
You can also say "was necessarily personal," but it could have a different meaning.

When you say "it's a personal choice," you mean that it's a choice done by a (particular) person, i.e. not done collectively. When you say "it's personal," you mean it is concerning the private life of somebody.

"Who is your girlfriend?," she asked.
"It's personal," he replied.

Answer 2

Yes, you could leave off the one (and also drop the article a) and just say that it was 'necessarily personal'. And yes, 'one' is being substituted for 'choice'.