No more than one backpack with about 2 pants and 2 shorts and about 5 t-shirts. Bring some cash with you in Dollars and you can exchange it as and when you need more Rupees. If you are from any country other than India you must get yourself malaria shots. It is a must. For all the Chaos that prevails, trains and busses run on time through most of India.
Transportation is no problem at all.
The overnight train is the best way to travel. Culture, Customs and Communication Through most of South India you can get away with just knowing English. Most people can speak English or even just understand the basics. People are very tolerant but when you go to a temple or church or mosque remember that your legs have to be covered fully. So wear a pant or a skirt that hits the ground.