Given that you're visiting India in the summers, it will be very bright (and harsh, both the light and temperature)
I think you should be able to get good results out of Velvia 50 thanks to the long and bright days (Disclaimer: I've never shot any 50 speed film, let alone Velvia)
ISO 200 films should suffice though. I've shot a roll of ISO 100 Sensia slide in mostly dim lights (sunrise, sunsets, dim-lit restaurants etc.) and the results are pretty good.
If I were you I'd be careful with the x-ray effect on high speed films...
What I gather from most shops here is that low speed films won't get hurt but higher ISO (800+) will die in the strong x-rays at airports (Again, I have no personal experience of this.)
Better buy some locally and if possible, get them developed before you leave...