Regarding compulsory registering agreement to sell

Dear Forum, I want to purchase a flat in Ahmedabad, Gujarat. My queries are as follows 1) Is it compulsory to register Agreement to sell (banakhat) at the registrar office if such agreement is without possession? 2) Is agreement to sell valid if done on 100rs stamp paper and notarized (without possession of property)? 3) What is the stamp duty for registering agreement to sale in Gujarat? Is this paid stamp duty liable for adjustment at time of final sale deed??