The advantage of golf cart batteries is that they are more or less indestructable. A standard lead acid flooded battery (starting battery) can take 10/20 deep discharges before its capacity decreases drastically (less than half stored energy) but a golf cart battery is designed for this service ie. +500 cycles. But they are 6 volts large, heavy, wet cells and leak nasty sulfuric acid if laid on their side. They are cheap though about $65 each at Sam's. Two to give 12 volts will be 120 +lb. and need water periodically and proper venting. Won't fit under the pass seat. The sealed AGM battery under the seat is 100 amp hours. A golf battery bank will be about 200 amp hours.
Bob