Great!
I would recommend a 2 battery system where one battery is dedicated to the engines (and starting them) and the other battery is the 'house' battery, for stereo, amps, etc. Blue Sea has an Add-A-Battery Kit (
http://www.amazon.com/Blue-Sea-Syst...918119&sr=8-1&keywords=blue+sea+add+a+battery). It has the switch and the ACR. I put that in my boat and it works wonderfully. Simple to install and even simpler to use. The switch has 2 positions that you use, basically: on and off. Even I can handle that (there is an emergency setting if you can't get it to start, but I bet you never need that).
So, the magic of that system is that in normal operation, it will keep your batteries and systems separated while discharging. When the ACR senses a charge on the system (like from running the motors or from hooking to a charger), then it will hook the batteries together so that both can charge. With our boats, though, the motor running only provides the barest of trickle of charge. But, the ACR will do whatever it can with what it is provided. If you follow the links on the Blue Sea website, you will find the instructions, wiring diagrams, etc. (there is also a fair bit of info on here somewhere, too).
Then you will need a second battery. If you want, you can get another one like the one you have (normal flooded lead/acid). Best price is probably at Costco or Sam's--just make sure you don't get an old battery (dates are printed on them). If you want to step up a notch, AGM's never spill, don't need water, and use charge a bit more efficiently. And are priced accordingly. You will need a battery tray (either just on the bottom or a full box) so you can secure the battery where you put it (batteries bumping about is a bad thing).
Then you are on to the charger. You will want a 2 bank smart charger. Minnkota is an excellent one, but expect to pay for it (oh, and I am pretty sure
@ChargerGuy does give a warranty with his chargers, btw, in addition to being about 50% off the new ones--not trying to persuade; just inform). The size you want will depend upon the size of the audio system you are planning on putting in. Basically, the more you pay, the higher the amps, the faster it will charge. I think most would recommend about a 10 amp per bank for normal sized systems (which should let you run for a day, put it away to charge overnight, and allow you to start with full batteries in the morning). You could get away with a 5 amp, but you might need a few more hours on charge to max out the batteries--so some of that depends on your use profile. And of course, you want to make sure your charger supports your battery type (most of the intelligent ones will do all types of batteries, but it pays to check).
So, I have typed a lot. How does all of that sound? Once we have your reaction to all of that, we can get to some actual wiring diagrams, which will let you better plan which types of wires you need, etc. Meanwhile, I expect others will chime in with their opinions, thoughts, etc...
