Elaboration on why switching speed doesn't usually need to be fast if the switching frequency is low: When the FET switches, it passes though a condition where there is both significant voltage across it and current through it simultaneously. A resistor of something in the range of 10k between the gate and source of the FET helps to assure the FET will stay turned off if power to the driver circuit is lost while there is still power for the FET and the load. Again, a resistor of a few tens of ohms in the gate circuit would be appropriate. If you don't need inversion, the 555 output can drive the FET directly. Be sure to use a decoupling capacitor of 100 nF or more very close to the gate package.
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