Yes you're right, it's not all that complicated or difficult necessarily. And yes, it depends on how you intend on using your configuration. Overkill is unnecessary.
Batteries have a life span normally in years when maintained and used as designed BUT are far from qualifying as perpetual motion devices. I've had cases where a battery might last more than 10 years and others where they lasted just a couple, despite proper use and maintenance. And, batteries can fail suddenly or even spectacularly thus I don't count them as life-saving devices where I'd trust my life to one.
A good single battery can more than suffice in most cases, DEPENDING on circumstances.
Keep in mind, any battery or bank of batteries is incapable of perpetual motion and thus requires some amount of attention, especially when electrical appliances such as stereos and such are involved. When 11 volts static condition is reached, that battery by rule of thumb, is considered 50% discharged and in most cases will require minimum 12 hours on a good automatic float charger at a maximum rate based on battery size to return to full capacity, best case.