You should definately buy a card.International roaming idea doesnot seems that effective.
The local sim will also give you increased connectivity for your tour so that you can have a hassle free experience.
You need to check if a single SIM works in all the countries. you don't want to end up buying new SIM everywhere, that increases the overall cost compared to activating international roaming. there are various service provider like matrix for which you can buy SIM in India and it gets activated abroad. check the plans and cost first.