so he's supposed to have 6000w with only two 800's to back it up? or he is seeing 9v. and if he was doing 150db it wouldnt be "decently loud" it would be chaotic inside.
what are you willing to do? do you want to wall your car? or would you rather just get the SUV? do you want big woofers, small woofers, tons of woofers?
for your car, a wall with two AQ HDC 15"s, one crescendo 3k, two deka intimidator g31s will put you at about $1500. the rest can go to wires, wood, etc. should do the trick. i would expect 148db out of that pretty easily.
if you want to spend more do the same thing but use DC XL 15"s and two 3ks. that should do a 150 easy.