Wsalam
Your plan seems OK for leaving early morning on Saturday as it will take around 7 to 8 hours to reach on Gorakh Top (depending on traffic condition and availability of Jeep at Wahi Paindi).
Yes, Cultus can easily go to Wahi Paindi as the road condition is good till Juhi. You may take a left turn from Juhi by using the navigation system as it will save around one hour of your time for going to Dadu and returned back to Juhi.
Room rent depends on the availability. If the place is empty than you can get a room within 2000 to 3000 (do bargain with Juman Jamali, the person who runs the restaurant and guests rooms).
Take a big can of water with you as the season is dry and hot up there. Take necessary precautions for the infants as the jeep ride would be a tough one for them (Suzuki potohar is the worst Jeep I've ever experienced). They may ask you around 6000 to 8000 but do bargain with them for 5000.
