Hello mate
I travelled in India for 10 months in 2004-05 and 5 of the last months on a enfield 350 cc i bought second hand from a fellow traveller. Here's a few of my thoughts:

- if its your first time in India, don't be in rush to buy a bike... see the country travel for at least two months first without bike, and then if you want to stay longer buy a bike. the train journeys in India are their own experience. and i would personally prefer the bike for the mountains rather than the plains.. but i only rode in mountains instead of plains so can't compare.

- if you go to goa or manali you can rent bikes there maybe in rishikesh too - renting is very cheap.. and once you are a bit used you can buy a bike if you want

- itinery - all the places i went to i found incredible.. these are the places and you can form your own itinary i havent been to rajasthan and many other places but at least this is a good start - you have to factor in rainy seasons, hot summers cold mountain winters etc...
south circle: goa, gokarna, mysore, bylakupe, hampi, (i didn't go to tamil nadu or kerella)
west to east (plains): calcutta, bodhgaya, varanassi, delhi
mountains: rishikesh, jashunath, Manali, Parvati valley, Dharamsala, Srinagar, Leh

- i am currently travelling in Colombia, South America, and have just bought me a Bajaj - Boxer 150cc. in colombia first hand is not that much more expensive than second hand, and i bought a first hand. I have met people who have travelled in 125 cc through the mountains in South America. So i expect 150 to be ok. ALthough in 2004 i got the impression for any long journeys in India it was only the enfield 350 or 500, but things may have changed by now.

- the number 1 rule in India for driving is that the bigger the car the right of way it has. As a bike don't take risks give way to the bigger cars.

- in india if a car shows you a headlight it means you wait there, I am taking this road first. In england incidentaly it means the opposite.

Good luck with your trip. There are plenty of bikers travelling in India, so it should be pretty easy to form a group.

Chao

Sameer.