absolutely essential; indispensable; vital; critical
bare minimum; minimum necessary (amount, number, etc.)
as necessary; if necessary; as the need arises; as occasion demands; where appropriate
necessary expenses; due charges; tax deductible expenses
necessary condition; requirement
necessary items; required items; necessary information; required fields (on a form); mandatory fields
necessary documents; required documents
necessity is the mother of invention
(socially) necessary labour; necessary work
necessary and sufficient condition
as necessary (needed); if necessary (needed)
(minimum) system requirements
necessary labour time (in Marxian economics)
minimum necessary; bare minimum; minimal requirement