Why is he called ‘Buddha’? What are the pre-requisites for becoming a Buddha?

Siddhattha was predicted by astrologers of his time to be a future Buddha (or Bodhisatta). He is called ‘Buddha’ because he reached supreme enlightenment (Samma sambuddho) at the age of 35. 

The pre-requisites for becoming a Buddha are fulfilling the ten Paramis, meeting a Buddha or the disciple of a Buddha, hearing the teaching (of the four Noble Truths) from them, grasping the teaching and then sharing it with others out of compassion.

What is the meaning of the word "Buddha"?

The word ‘Buddha’ is not a personal name or title, but refers to the attainment of supreme enlightenment by an individual after a prolonged period of spiritual evolution during which the 10 exalted spiritual virtues (Paramis) are perfected. 
In the present aeon it refers to Bodhisatta Siddhattha who became the Buddha Gotama at the age of 35 years. 
Historically, we are also aware of 28 Buddhas of the past, and the future one who is called Lord Metteyya.

Is the word ‘Buddha’ a personal name or title, or does it stand for an office?

The word ‘Buddha’ is not a personal name or title, but refers to a state of enlightenment. In the case of an Arahat or a Samma sambuddho, it might also be considered an office to the extent that these types of enlightenment include the task of passing on the Dhamma to others (gods and men).

The Meaning of 'Buddha Vandana'

Buddha Vandana, or ‘worshipping the Buddha’ is one of the devotional practices performed daily by Buddhists. It is done as part of a reverential recollection of the Triple Gem, consisting of the Buddha, the Dhamma, and the Sangha. In this way, followers build their confidence in the Triple Gem. The Buddha encouraged such recitations in times of fear or disturbance so that these can be vanquished, since the Buddha is free from all kinds of defilements, like greed, anger and ignorance.
Buddha Vandana begins by recalling the supreme accomplishments of past, future and present sammasambuddhas. It then continues by making a statement that the Enlightened One is a person’s sole and supreme refuge, and that one wishes to attain enlightenment through taking refuge this way. It concludes by humbling oneself before the holy feet while asking for forgiveness of any transgressions, and stating one’s continued reliance on the Buddha until reaching enlightenment.

Together with the Three Refuges, what moral principles should a Buddhist follow?

A follower of the Buddha should also follow the moral principles set out in the Five Precepts. Namely they should abstain from killing, stealing, sexual misconduct, lying, and using liquor, alcoholic drinks, or intoxicants that cause heedlessness. They do this by acts of loving-kindness, generosity, contentment, truthfulness and mindfulness.