You would only use two of the 6 volt models in series for the stereo bank which would equal 12 volts. The starting battery could be a conventional marine Group 24 12-volt lead acid.
The only reason to avoid golf cart batteries is if you have already had a history of back problems.
Depleted amp/hour capacity will be the same issue relating to the alternator and shore charger load as with any other batteries of the same capacity.
The dimensions are a consideration. Golf cart batteries certainly are a great amp/hour per dollar value.

David
Earmark Marine