What sub to get?

If you need that kind of small size enclosure I would HIGHLY recommend looking at Image Dynamics IDQ series. 

http://www.cardomain.com/item/IDSIDQ12D2V2

They were designed to fit in small enclosures and have great SQ while doing it.  They don't require a crazy amount of amplifier power. 

That said, they are more SQ drivers than SPL drivers.  500w of properly set gain is all I'd ever look at throwing at these drivers. 

.... or the new Brahma's when they come out.  Both the SQ and the output will be excellent.

^^^^^ Exactly. This vehicle and the needs of the user dictate a custom built box for a specific sub or subs. The sub or subs will have to be chosen based on the ability to fit into the space available. A sealed box is very forgiving on any sub you put into it. In some cases the el-cheapo Wal-Mart special will outperform the expensive brand name sub. Bass response is more about the box than it is the sub.

There is no way that I would try and do 2 12's in this vehicle (save for maybe the Pioneer Premier 12" shallow mount sub if he has to have a 12"). I would find out from a custom box builder at one of the local shops as to how much airspace is available under the seat and which direction to fire the subs, up into the bottom of the seat, down into the floor or forward into the cabin (this being the best use of precious space). A stellar performer in a small enclosure is the MB Quart RWE254 in a 1.25 cu.ft slot port enclosure.