What are your goals? What type of music do you listen to?
Capacitors? Save your money. If you're not wanting to halfass the system (and I use "system" very liberally here, because this is NOT a system... it is a sub-stage!) you're probably going to be looking at dropping into a single sub, so you can afford the proper items to go along with it... Decent amp. Appropriate wiring. Possible alternator upgrade. Proper enclosure. etc