Hey Ian. I don't know how much you know about subwoofers, but the type of sub and enclosure really depends on what type of music Cody listens to, almost more so than any other type of stereo component. I say that you can never go wrong with JL Audio subwoofers as they're a good all-round unit, but there are some that sound better and others that make more boom.
Whatever type of sub you get, be it new or second hand, be sure you are also able to get the thiele-small parameters for that sub so you can build a proper enclosure. Usually two or three enclosures are suitable for each type of sub, and you'd choose the one best suited for the application.
A simple sealed box is usually very small and is less sensitive to "errors" in size, but needs to be fairly air-tight to work properly. It's the quietest and least efficient, but usually produces the smoothest bass response and the tightest sound, generally preferred by audiophiles.
A ported box is usually about twice as large (internally) as the sealed box and has more tuning features. It's less sensitive to having all the gaps perfectly sealed, but the entry and exit around the port must be nice and smooth to avoid port noise. It's more efficient than the sealed box and makes more bass around the 45-60 hz area, but it should not be used below the port tuning frequency because then the excursion of the cone goes out of control. Built properly, this type of sub is actually cleaner sounding than the sealed box but is not quite as crisp on transitions, and of course it can't play frequencies down to the subaural zone like the sealed box can.
A fourth-order band pass box is more efficient than both the above boxes, but that comes at the penalty of complexity and sound quality. One of the bonuses of the fourth order is that it doesn't REQUIRE a low-pass crossover because the sound on either side of the operating band is greatly attenuated, but it will always sound better with a crossover. They're sized somewhere in between a sealed and ported box, though sometimes they're bigger than ported. Like a ported box, the usable frequency doesn't extend down into the subsonic range, but because the speaker is sitting in a sealed enclosure it's not as damaging to the driver as it would be in a ported box.
There's also a sixth-order band pass box that's even more complex and limited, but probably not worth talking about. There are transmission line enclosures like what would be used in a home audio environment, but these are generally huge. There are free-air subs as well, but they require a trunk where the cargo and passenger compartments are perfectly sealed, and you need to mount the speaker facing towards the cabin (usually on the parcel shelf, if you have room, or in an adapter panel below the parcel shelf if possible.)
Have fun
