Personally, I've never been a fan of Infinity subwoofers. I have two of the HE2 10" subs and really like them. If you are going to buy the speaker seperate and then buy a box to put it in, the 12" HE2 is the way to go.
However, I'd actually consider buying a Type R that comes factory mounted in the box. The box is made to produce the best possible sound from the sub, and REALLY bumps. I almost got one myself ---> http://www.smartcart.com/alneds/cgi/display.cgi?item_num=SBR122CR
If you shop around, I'm sure you can find a mounted Type R cheaper than that online.