For the cables you want to keep the same size as the factory put in or go Bigger, never smaller.

For cables from the alternator to the switch system to change it you will always want to run 2 gauge or bigger.

Ground cables need to be the same size as power cables.

As for batteries the 650 is plenty big as a starting battery. If your going to a second battery it would be the bigger one for the stereo.

As i understand or have been told you want to stay with similar batteries in these setups, or maybe its same brands. You don't want to run a Wet cell Deep Cycle with a Gell Cell Deep Cycle.

Im no expert in batteries this is just how i understand from the other guys on here.

I do know that Power and Ground always need to be the same because those are your voltage pipelines and the return path needs to be the same as a supply path size.