I'd say the minimum you'd need depends on your experience and skill level, as well as what your goals are.
In general I'd say the minimum would be power compact lighting and 1 or 2 small powerheads (depending on the size of the tank), some live rock, enough sand to cover the bottom of the tank, a heater of appropriate wattage, a clean up crew, RO water, a heck of a lot of patience for a few weeks until the tank is stable, and finally some shrooms.
