What is/are 'banns of marriage'?
Public announcement of the upcoming wedding in the church. It's not just for Catholics. Churches differ in how this is handled, so she will have to find out what is required in her church in Poland, and you will have to find out what is required in your church in India.
Where can we get Letter of Intent and Letter of Acceptance?
Your lawyers will handle this, preferably about a year before the wedding.
Church Marriage Preparedness thing you said?
Some churches, like the Catholic church, for example, require you to take a course together about married life before they will agree to marry you. If her church does, you'll need a certificate for that, and also probably other church-related documents like proof of baptism, confirmation and active participation. You'll also need a certificate of no impediment, both for the church and the civil authorities.
I don't think my visit to Poland will be more that 10days.
You have to wait at least thirty days after applying for a marriage license to actually get married. Also, if she is Catholic (like almost all Polish girls are), and you are not, there is a long waiting period for getting married in the Catholic Church, most especially for mixed marriages. There are A LOT of hoops for you to jump through, and they are not going to make it easy for you.
If she is not Catholic, then she and you have to find out the rules for your respective churches. Talk to your pastors.
Also, the wedding is five years away. Your lawyer will be able to answer your questions in your particular case, or refer you to someone who can. You'll both have plenty of time to learn about the process and gather the necessary documentation.