I like the momentum of the thread.. just to know the feel of how even sub-200kmph, have a look at this video.

Handling is the issue, when speed catches up, the controls become more and more responsive/sensitive. so difficult to control the handling of the car. the center of gravity has to be perfect, the weight transfer in an event of acceleration, braking, cornering has to be spot on. As Iyer sir said, you will need a special test/run surface without any imperfections and most importantly, no traffic [human or vehicle] to have a safe run. Keep the thread alive with the progress of your project, and you will definitely get the necessary pointers for it. all the best!!

As suggested earlier, try a tethered car, which can be run on a basketball or tennis court.
It will postpone steering related issues for later, after you get the car running!

Followers of this thread may like to see AnjanBabu sir's thread:
http://www.rcindia.org/cars/(build-log)-3-wheeled-rc-car-sweepstakes-2014/msg186345/#msg186345

all right...
i've found some top rc hobby sites viz. hobbyking,bananahobbies,rcplanet,hobbytron and many more....
which one is the best to start from scratch???
(i feel like they are too expensive).
 btw i hate online shopping....
are there any shops in mumbai or thane where i can get the parts at lower cost??

there are in mumbai but all are online. but I doubt there is any car related thing in mumbai, I am not sure. but u ll get the motors n escs in mumbai.

Hobbyking cheapest option for beginners.

Testing is a very important part... I got myself a thunder tiger go kart, and put a 2200 3s on it... Was trying to see how fast I could go and I hit a small stone. A stone. 😐
The car just took off. It went spinning into the air and rolled across the road.. Broke the servo control horn so now can't drive for sometime 😕
And this wasn't too fast, but even something like a small stone can be dangerous for a small car going at 100+ kmph...

+1 to that. What you might consider smooth tarmac will be a crater field for your small rc car. Check out the Traxxas XO1 videos. Look where they do their speed runs

High gearing will work. You may need a bigger motor.

 Brakes can be applied using brushless motors. No need to run the motor back wards, the speed controller will take care of it.

Torque and other characteristics of brushed and brushless are somewhat same for the application you are plannig just that brushless is less maintainence since there is no contact brush to clean. You may actually be better off using a brushed motor since the esc is much simpler. You just have to control the voltage to the motor.