You're all over the map there bud. The W6 is arguably one of the sweetest sounding subs on the market. The HDC3 is a solid cheap SPL sub. The SD 2.5's are more SPL based then SQ.
So what it comes down to is if you want SQ, the W6's everyday of the week. If you want more SPL than SQ, then I would choose the 18" HDC3 if you have the room for a proper box.
My brother has a Cherokee like yours and a single 12 dc sound was much louder than a dd 3518 was. So skip a single 18. Id do two audioque 2.5 12's if you want loud. My brothers using 15's now but they are in a small box for 15's Put the 12's in a subs up port back design. Actually personally id look for different subs to get more low end instead of the audioque's. As a side note my brother hit a 143 and change on the dash with a single 12 on 1500 watts in his Cherokee using a daily tuned box.