My advice is to purchase Apple Watch in the country in which it will be used, particularly if you want the cellular version. USA and India use different cellular frequency bands so the cellular functions of an imported watch are likely to be incompatible. It is also highly likely that an Indian cellular operator would be unable/refuse to activate on the cellular network, because the device is not a model which has type approval for India.
The non-cellular model should pair and work with any compatible iPhone (iOS software version limitations with older phones) but you would not have an effective warranty cover. If there was a problem you would probably have to return the Apple Watch in the country in which it was purchased.