The ability triggers when the spell is cast, so it becomes a creature before the spell resolves.

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

When it turns into a creature, it is no longer an enchantment.

It changes into a creature even if the spell is countered.