What is the output stage of the voltage sensor? If it is a reasonably low impedance like an opamp or even an optocoupler, then one output can drive two circuits like the FET-A gate and a delay circuit. Why do you think you need a buffer section? From your descriptions, it looks like the voltage sensor output could drive FET-A directly, and it's negative edge could trigger a 555, so I without more information I don't see a need for a buffer, and without more information I can't design one anyway.