I don't know if you said this earlier, but what are your max dimensions you're willing to give up for an enclosure?
We can offer all the advice in the world but it would be for not if the shit didn't fit lol.
Also, do you know the amp rating on your alternator?
For a good estimate I would say you'll need 200-250amps of current to power your amps without too much of a voltage drop. At least two khc2400's or two d3100's would be needed IMO.
So, you're looking between $5-600 for batteries. $4-600 for subs(if you go with two). $3-600 for additional/more powerful amps. All told, you're looking at $1500-2000 for equipment/materials. Now this is all assuming that you buy everything new. Obviously the cost will be cheaper if you go with used equipment.