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"?
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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.