Well, If I were to go with a long straddle, i would wait untill the last 10 mins. If Nifty closing likely at 7800 then I would buy ce/pe 7800 strikes. if closing towards 7900 then I would consider 7900.
Yes qty should be equal.
Obviously lower the IV the better. You will make money when the IV shoots up.
Note: I'm not suggesting that you should opt for along straddle. Taking the position is your headache. Above reply since you asked a question