1. No, the unregistered sale agreement is not valid and canot be enforced in law.
2. Unregistered sale agreement cannot be enforced in court of law hence how much ever value you may buy the stamp paper, without registration it may not be legally valid.
3. Yes, it is mandatory to register the sale agreement to make it legally enforceable in court of law, the registration cost may be enquired ion the local registration office.
4. There are lot of terms and conditions to be incorporated in the sale agreement which cannot be mentioned in a line or two, you may approach a local document writer who has a template or set of format for this purpose.