Cellular devices intended for a specific geographic market are often optimised for that market. While the majority of cellular radio bands and Cellular channel frequencies are consistent around the world, some countries have additional cellular bands that are not used elsewhere. WiFi is generally ubiquitous - although some 6 GHz WiFi bands are not available in many countries.
As such, a current Cellular model iPad purchased in the US will be "optimised" for the US will be usable Worldwide, but may not cover all the available channels used by your Indian Cellular Carriers. In most situations, any difference in band-coverage will not significantly impact the usability of the iPad with the local Cellular Operator. Consider that Cellular devices are designed to work while travelling between countries - and/or to "roam' between networks and across national boundaries.
If making a new purchase, consider the warranty implications. Should you ever need to seek warranty assistance, in some circumstances this will only be available within the country of purchase.
Also consider that there is very little actual cost differential between devices purchased in the US and other countries. US prices are exclusive of purchase taxes - which must be added to the price paid. Prices in other countries often include local purchase taxes. Also consider that you may have to pay duty and local taxes upon import to India.
It is generally recommended that you purchase a device in the country that you spend most of your time - as this simplifies both purchase and warranty - plus you have a device that is optimised for your country/region.