Well Dippul, that was sure alot of questions all in once... I'm no admin here but next time you might want to create problem specific posts like: "I need help with calculating motor torque" or "Need wide angle human detector for robot".
Ok, on to your questions:
You have problems with getting enough torque if I'm understanding you right? You also have a problem with too much rpm? This sounds like the perfect combination for a gearbox! The gearbox will take your rpm and make it into torque instead. What you have to look at is the rpm/torque demands. How fast does your robot need to be and how strong?
For your human sensor there's probably hundreds of ways to go about it. What I found in my local dealer was the Parallax PIR sensor. Don't think that will work if you put it on a mobile robot though... But if the bot is to operate in a smaller space you could statically mount it to sense the whereabouts.