Cooler Master GX series is crap, including the new GXII units.
If you're only interested in 80 PLUS Gold certified units then why are you looking at Cooler Master GX? It is only 80 PLUS Bronze certified after all.
HEXUS has tested the card listed in your system specs. If you look at the power consumption page, it shows the whole system drawing 225W under Far Cry 3.
http://hexus.net/tech/reviews/graphi...ual-x/?page=10
I think, even with overclocking, the difference in power consumption between HEXUS' system and your planned system, a high quality ~500W PSU would still be more than enough.
If you want to be able use a second R9 280X, then 750W is fine.
I only know of two online shops in India (flipkart & theitdepot); after consulting them I can't see anything besides the Seasonic S12G series for the cheapest, high quality 80 PLUS Gold units available.
If you don't mind 80 PLUS Bronze certified units then Seasonic M12II series (which are definitely loud) and Cooler Master GM series are high quality units which are cheaper than Seasonic S12G units.