1) XiaoMi only .
2) RM600 or less .
3) Doesnt lag much even using for a long time .
4) 4.7" to 5.5" .
5) normal tasks + little light gaming .
6) buying for my dad .

I have had good experience with nexus 4 coz stable OS and doesnt lag much even using for 1year plus with many things installed. Heard and know that XiaoMi phones are best bang for the buck , just dunno how their OS goes after long time usage .

now considering RedMi 1S , RedMi Note , XiaoMi3 (used) .