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You do not need a second battery. They add complexity, weight, cost and are not necessary unless you are running a bait well etc for hours on end. A properly sized marine battery is sufficient. They are just trying to sell you more stuff.

Make sure the dealer mounts the engine AT LEAST 2 holes up. You will want a stainless prop but it may take a few tries to find the correct size.

Garmin GPS have an option to display voltage, check your unit.