This is my solution:
The system is made by Helios and is made from these components.
The system is *very* silent as the fan is capsuled and there is a noise silencer built in. It is nothing which you want to install yourself as there are some pitfalls which generate noise if not handled correctly. You'll need a louver with dust filter somewhere.
Building a ventilation into a darkroom is easy. Building an efficient one which does not make noise is very difficult. Fans used in toilets are inapt as you stay in a darkroom for hours not for minutes.
My vent transports 380m³/h at full speed. This is about 15x the volume of my darkroom. Usually I run it on step 1. It is practicallly noiseless then. On step 2 it is barely hearable. I can run it conveniently up to step 3. Step 4 and 5 I rarely use. On step5 it is appr. as noisy as our kitchen vent on step 1.