To answer your first question, Yes, they both can get married in India irrespective of the fact that their religion are different.
There marriage will be solemnized under the Special Marriage Act of 1954.
and yes there marriage will be official and valid.
Regarding your third question about the documents that are required for marriage are:-
- Birth certificates of both the parties (for age proof) as in India the valid age for marriage for a boy is 21 years old and for a girl is 18 years old.
- A valid visa of more than thirty days for the foreigner.
- An affidavit signed by both parties declaring their single status.
- An address proof and passport size photographs of both of the parties.
- Adequate documentary evidence showing thirty-day of residence in India.
- A ‘no-objection’ letter:- a citizen of any other foreign country is required to bring a no objection letter from the Embassy or Consulate of his/her country. The parties also have to wait at least 30 days from the date of initial application to formalize the marriage so that the marriage officer can publish a notice, which might even include a newspaper publication for giving an opportunity for any objections to the marriage to be voiced.
I hope I was able to help, however this answer of mine is purely from the internet research so I would suggest you to consult a lawyer and clarify everything.