1. It is legal to buy a laptop from abroad.
2. There is no custom duty if u bring a laptop from another country with u... however if its bought online... custom duty may apply... not sure.
And what site r u planning to buy it from? The lowest prices on the net are mostly at newegg.com, but they dont ship products to India. However you can visit newegg.com and compare prices to the site that ur buying from.