your going to need a dual 4 ohm sub. that is capable of 1200 watts rms. so you are going to need a decent beefy sub. I would suggest a decent 12. with the kia sorrento you dont' have a big area in the back. I would suggest a decent 12 in a ported box but a smaller ported box tuned a little higher. Doing say a Power Acoustik Fubar 12d4 its going to be a little over power. But I've personally ran 1000 rms to this sub all day long. if you gain down the amp a little you will be a little better sound quality. and still get loud. I would build a box around 1.8 cubes tuned at 38 htz. That will be low enough to be very clear down low and very sq. but it will allow you to get silly loud and still get clean accurate bass. And doing a box a little smaller than spec will allow you to run a little more power to the sub than rated with out it blowing the coil. These subs are great. I ran 2 12's and i've run 2 15's as well. I am considering running 6 of these exact subs in a wall. check it out not a bad price at half your budget.
http://cgi.ebay.com/1800W-POWER-ACOU...item35a41654cc