In need of counselling regarding gmat..

Hi. I am a mech.engg student pursuing my final year in India. My aspiration is to do mba abroad and I am working towards the same.
But I have certain doubts regarding gmat. I hope someone helps me in clarifying them.

1) I have decided to take my gmat on sept 15.
How long will it take to receive my gmat score. Can I postpone my gmat date or will it affect me from applying to major reputed universities.
Will it be too late to apply for top50 universities if I take my gmat in october?

2) Some people say you need to have work experience in order to apply for gmat. Is that true??
If not, how good is the opportunity for freshers doing mba abroad ( especially USA). Is there a good chance for freshers getting admit there? Is there a better career opportunity for freshers after completing mba in USA?

3) Can you please let me know the details of top universities offering mba for freshers.
My area of interest is Operational Research and General management.
And how much will I have to score to manage getting admit in a top b-school??
My cgpa is 8.74/10. Is that decent enough for a better profile

I would be really grateful if someone clarifies all these queries as soon as possible..
Thanks in advance.