Whats the Currency ? Is the use of Credit Cards widespread in India ?

Indian currency is in Indian Rupees (INR). Its always a good idea that you carry minimum required cash during your travel. Plan early and pre-book your Air Reservations, Railway Reservations, Hotels and Surface transport so that you need not carry hard cash. Most of the big Hotels and Shopping Centres in India accept credit cards.

It is wise to use Public Transport and hire a v ehicle to explore the destinations?

Public Transport vary from Good to Bad depending on the area you are traveling to. Its always a good idea to hire a car so that you travel with comfort and save on time too. Should intermingling / interacting with common Indian people be on your cards, you can undertake a part of your journey by Public Transport and rest by your own vehicle. You may also travel a leg of your journey by Railways where you will get an opportunity to interact with fellow travelers who would invariably be hailing from different parts of India. Wide range of vehicles are available across India. Depending upon the number of people traveling, vehicle can be chosen.