You have it ok. I would go with a Starter battery for the "starting" and a Gel battery for the other side. You can go with a Optima for the best bang for the buck or if you want the best with out cost considerations go with the large Stinger battery. Make sure if you ever charge Gels with a charger you read the directions about that so you do not ruin the battery. You should run a ground btwn the two battery's yes. Make sure you use TINNED wire on everything to avoid issues with Corrosion. The Perko switch will work fine. You just switch it to the off posision to listen to the stereo. When you start ONLY go to the start battery then swich over to BOTH, NOT to the other battery. If you just go the the deep cycle you will put a much added load on the alternator and could cook it.