$800 gives you a whole lot of options... are you looking for just a sub/amp/box or are you looking for some front stage speakers as well?
(2) 12" VS (1) 15"
the 12s would probably be louder... but it depends on what your looking for and kind of music you listen to
you should get a measuring tape and find your MAX dimmesions in your trunk so you know what will fit... you dont want to buy a 15" and find out later it doesnt fit
also the audio stuff you listed eariler dont sound like there worth much.. never heard of any of them, like the other guy said try pawning them or goto your local highschool and see if you can sell it to some kid for more than there worth lol