Nobody Really Wants To Heal

[Ilyrian]

Secondly I've done the research, the game offered more flexibility and customization with abilities, skills, and attributes as opposed to how it currently works today. You can be a mage of ice arcana and fire instead of being forced down one land.

Already been covered but a world of no to this.

If anything Vanilla WoW was much more rigid in what worked and what didn't - from the classes you picked to the spec you had to take.
I played a full time Holy Priest back then and though I loved it (and I will be going back to gets some nostalgia feels) there was no choice, the build was set, unless you were unlucky enough to be the Prayer of Spirit bot, and you did your job.
Horde choices for healer: Priest. Occasionally you'd see a Druid or Shammy.
Ironically modern WoW is more flexible especially where class and spec choices are involved.
(Unless they are going through a loving Discipline Priest phase where all other healers become irrelevant - though this hasn't happened for a few years in raiding to be fair).