Where is the best place to locate the motor for my ceiling unit?

For ventilation purposes it does not matter whether the motor is installed on the cooker hood, in the duct or at the end of the duct. The location mainly depends on the installation space available.
If the maximum installation height is 35 cm, you can keep the motor on the cooker hood. If you do not have this much installation height you can opt for the Pure'line Compact or the cubic remote motor.