You are supposed to have a picture as well as adahar card for registering the property into your name. Power of attorney was used to the real estate matters. However, there are many cased it has been misused. There are incidents like the real documents was not even registered in buyers name etc.. After all, it all depends on who is going to handle this matter for you.
Power of attorney has to be certified by the Indian embassy.
If you are a US citizen, you can buy residential property but you are not allowed to buy agriculture land in India. (OCI or any other form of visa will not give you this right)