You should really only need to upgrade your battery's wiring. for the equipment you list a cap or a second battery is really not needed.
That's it. It should cost you about $20 worth of 4 guage welding cable, some ends and your car up on jackstands for 10 minutes while you attach everything.
Do the battery ground cable (battery negative post to body)
Then the alternator cable (battery positive to alternator)
Then the engine to chassis ground (engine to body)
Flickery lights will go away completely.