12 volts total, and big enough to ballast your boat. Model Power and Component Shop are the best suppliers around - go have a peek at their websites. If you must run two 12v batteries in parallel then make sure they are the same capacity and type; better to use 2 x 6v in series instead.
For prop size I'd prevail on Mr Shipmate 60 - he's the acknowledged Forum Wise One ref matching whirling things to each other.
Me? I'd go for a 4 blade x 50mm to start with.