Hi Raghav21oct,
To start, the 700+ Score is approximately the 90th percentile - so clearly most people will not Score that high when they take the Official GMAT. By extension, no GMAT product can "guarantee" you that type of result. That having been said, the Official Guide is a fantastic source for realistic practice questions - and the relatively low price and convenience of the book make it a resource worth having. As an aside, many Test Takers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so if your studies are going to be limited to 2.5 months, then there might be a limit to how much you improve in that time.
Since it sounds like you're just beginning your studies, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can take 2 for free at
www.mba.com (and they come with some additional practice materials). If you want to do a little studying first, so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can discuss the results.
I'd like to know a bit more about your timeline and goals:
1) Going forward, how many hours do you think you can consistently study each week?
2) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich