don't buy a mobile with the an of getting an update. Looking at the way android mobiles are receiving updates, don't expect JB update before February '13. And as by that time S Advanced will be replaced by S 3 Mini or something like that, update may get cancelled also. Your best bet, buy a mobile with Android 4.0 and don't expect any further update. If there is an update, take it like a free bonus gift.
Under 20k check Motorola Atrix 2 & LG Optimus L9. Both have best hardware under 20k and also big display. Atrix 2 won't get Android 4.1 update Motorola announced, L9 should get update by Q1-Q2 2013. Thats why i said, don't expect or wait for updates. Your next bet is Sony Xperia P at 21k from adexmart. Lastly, updates are free. Won't cost you anything neither raise the price of the device.