id travel with assorted bags vs one large one, ive seen to many backpackers haul there packs and suitcases into fine dining restaurants...600$ for 3 weeks is a little scant, id go with more moeny, india is poor and things cost less but its not like youll get a bunk in a hostel for a quarter... accomadations are pretty cheap, if you find hotels too costly opt for a reputable "guest house" they are pretty much very small hotels (or very large bed and breakfasts). i would avoid spending to little on a hotel (under 25$ per night) since this may entail a room with no ac...

you can even book a guesthouse room via expedia, just search for rooms in any indian city and then organize by price..expedia may net you a more reputable guestohouse with more to lose if they screw around...

AC is key...during that time of the year accept nothing unless they have ac!!!