It really depends on what time of music you listen to and what your budget is. For most people in the KC 12's are overkill unless you only have 1. Unfourtnatley due to the limited space under the seat it is hard to make a box big enough for 12's unless you do a seat lift.
Tech12volts is the resident box builder, he is a site sponsor and does some real quality work! Check out the recent post by P'dizzle and you can see one of his recent builds using Sundown subs. You wont find a box on ebay that is even worth the price of shipping (unless its a T12V box!) Most of those are premade junk. All of the boxes James builds will be for the type of music you like and the subs you plan on mounting in them.