I think you have to extend ur budget near 60-70 lacks to buy a kanal plot in green area nearby main Islamabad city.
- Bani gala. As u mentioned. (to live there u will have to fully secure urself. I mean u must need a guard or driver to live there).
- CDA announced Park enclave which is in green area and near to hills but highly Expansive. (a kanal plot for 12 millions)
- Bahria Enclave Islamabad(First project of Bahria town in Islamabad) which is more nearer than bani gala. and 2.5 km far from Park Enclave and it is situated in very green area and it is more nearer to hills than Park Enclave. Plots are in very reasonable price. There u can get a kanal plot for 60 lacks.
But it will need 2-2.5 years for development.
Same as the case with CDA'S Park Enclave.
You can also Check CDA sector D-12, E-12. Which are nearer to Margalla hills. But plots are expansive there( kanal plot above 15 millions).