Power rating of H-bridge
The power rating of the motor is for the input power it can handle, 18A*24v =432 watts.

The power rating for the H bridge is for the loss power its packages can dissipate, which has nothing to do with 432 watts directly.

There are two main loss mechanisms in the switches of the H bridge

1) Voltage drop across the FET resistance when conducting with a low voltage drop, conduction losses
2) Momentary high power dissipation as they commute, switching losses

Look at the datasheet static RON specs to see what the conduction losses will be at your currents. Look at the data sheet dynamic switching specs to see what the losses will be at your current, voltage and PWM frequency. Add those together to get the total losses. Look at the datasheet to see what the thermal resistances are, to see whether you need a heatsink, to keep the junctions cool enough, at that power.

Generally, if the H bridge is over rated for the current, and for the voltage, and you use a PWM frequency within its suggested range, then you will be able to handle the loss dissipation, and you will probably need a heatsink. Whether it needs to be fan-blown or not depends on how big you can tolerate it being.

If you don't want to do the loss sums, then start with a humungeous heatsink, and reduce the size while checking the temperature.