Well borther, I would recommend you not ot go for a perparatory course.

This can be a huge burden for you. As your low scores (which you also know), shows you have almost no command (right now) in physics, maths and chemistry.

So, I going to IIT will only demotivate you and this will do only harm not good.

I would recommend you to opt for decent NIT or state engineering college. 

Though final decision is always your's

Thank You