For 1 thing. I don't recommend having two different brands of subs running off different amplifiers. Sound quality will not be as good.
You most likely will not need a new battery until you test it all out. It should be able to handle the subs but don't exactly know. If your battery does die after installing the subs, then go with another battery. But keep in mind, you should also get a high powered alternator to handle the extra battery
The capacitor is not needed so I wouldn't waste your money. Only get one if you notice your lights dimming when the sub hits.