Pay half your life, rounded up: This enchantment becomes a 4/4 Phyrexian Horror creature with flying.
When it turns into a creature, it is no longer an enchantment.
If you have zero or negative life, half your life is zero, so it costs nothing to use the ability.
A noncreature permanent that turns into a creature can attack, and its
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.