1) Brand New: 32k, Used: 30k (MAX) - 26k (Minimum)
2) Sometimes, white ones are a tad more expensive because they're rare.
3) Not that I know of. Just international Xperia S: LT26i and American Xperia S: LT26a. Both work in Pakistan as US also has GSM carriers.
4) Sony phones don't have warranty in Pakistan unless you get it brand new from HSN.
Good Luck and post pictures when you get it.