Hello hemanth04!
If we buy AVP from US, is it legal to use in India for corporate business and R&D purposes?
Hmm... I am not 100% sure if this is right, but before you do so, you might have to Change your Apple ID country or region. Also, Apple Vision Pro is available in the US or ordered from apple.com. I don't think you need to be in the US to order it. You should probably ask this question to an Apple Specialist on the Buy Apple Vision Pro page.
Is the gadget ready for developers with SDKs and ARkit kind of plugins availability?
I'm glad that I can simply answer this by saying, yes, they are available via Xcode. In order to develop for visionOS, you need a Mac with Apple Silicon. You can learn more on the Apple Developer visionOS page. You can also see the visionOS Release Notes on the visionOS Release Notes in the Apple Developer Documentation.
I hope you find this helpful!