Wait how did you go from wanting a 12" $100 sub to an 18" sub? You do realize that 18" subs are not cheap, and if you want something to make noise your going to be spending close to 300 or 400 dollars on the sub?
But hey if that is what you want by all means, I am currently shopping around for an 18 atm as well. So far I'm really liking the DC level 4 18".
Are you going to be building this box yourself?