If Russia can sell us , we will buy it . We may go directly for s500
Considering the fact that the delivery of S500 systems to Russian army is postponed to 2017, you might have to wait for at least 2020-2022 to buy S500 systems from Russians if you want to go directly for S500. Even that date is not certain and will depend on the capibilities of competitor missile systems and the geostrategic balance of the region.