Under Sunni Law, which of the following is an essential for a valid Nikah ?

A marriage within the absolute prohibitions of consanguinity, affinity or fosterage is void (batil), so absence of absolute prohibition is essential to a valid nikah.

The witness requirement can be met by one male and two female witnesses, and its absence makes the marriage only irregular. Payment of dower is not a condition of validity: a marriage without any fixed dower is valid and the wife is entitled to proper dower.