BOLIGVENTILASJON / Balanced ventilation

Hello there, Need help to install a Balanced ventilation system in 2nd floor of a house. There are 4 bedrooms and 1 bathroom in 2nd floor, about 80-90 square meters. Above them, it is a empty loft where the ventilator can be stored. Kindly advice the required devices, time, and cost of installing. i am living in 1365, Blommenholm. Thank you