well for india,the biggest criteria for purchasing weapons is better(or even best) technology.Russians have really improved their tech and they are cheaper(and sometimes better) than the western tech.
and anyways india has the option to buy from any nation it chooses(except china ofcourse).
And regarding relations,china and india definitely have better relations than what they had a few years ago and their trade is improving.But trade always is not a major indicator in good relations.
take for instance
1)china-usa trade=$210 billion
2)china-japan trade=$240 billion (somewhere close)
Trade is imp. Relations with china were much worse in the old days when there was little trade.