buy the right car first (panelvan, wagon, van, etc) then turn it into a giant sub enclosure. remove flex from all panels, seal everything up, with a shitload of expanding foam, sound deadener, bog and plywood...
then, fill it with the biggest subs possible, a crapload of high power amps with the right amount of power for the subs, good dozen of deep cycle batteries to provide the current, and don't forget all the zero gauge cabling