Who investigates possible impacts of legislation to prepare bills to be considered by the house?

The committees

Explanation:

Congress is divided into 2 chambers. The House of Representatives and the Senate or "Upper House". Each of these institutions is responsible for passing Laws that have to be approved by the other in order for them to reach the President's office.

Each House has a group of committees. Committees are work groups that oversee a particular aspect of the legislation. Representatives who work in these committees are in charge of analyzing all the related bills and voting on which of them should pass to an actual hearing in a regular session with all representatives.  

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.

"the attempts of the proletariat to rise above this abasement"