To sum up everything: the system is up and running for about half a year.
I really like the result. During the coldest days we had just a few centimeters of fog (not even water) on windows in the bedroom in the morning. All windows are clear: no water, no mold.
Almost everything was bought at the company "Smooth Air". They have all their catalog on the website with prices. I bought there: fan, heater, filters, ducting, joiners. Diffusers were from a different shop (a lot cheaper and the color we liked), air valves I got at Aliexpress.
Total price was arount $1,800 (incl GST).
I've created a module to read the temperature and humidity in the attic space, after the fan and heater inside the duct. It also controls fan speed, opens and closes air valves (to be able to control the direction of the airflow and to blow to specific rooms only).
As I can do everything with the system - I've created a few complex schedules to keep it in the way my family likes it.