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Thunder, for your usage model I would definitely suggest powering to the maximum horsepower; that's 150 for the Outrage version but I don't know what it is for your Guardian. Twins may give you greater maneuverability, but are a lot more expensive, and overkill for an 18 especially as I don't think you'll be going offshore in Arizona.

There are many good choices for 150s - be sure to get a modern, fuel-injected motor, and you'll be happy with good mileage and a quiet ride. Avoid the older Mercury Optimax - you might save a couple of bucks, but they are noisy and high maintenance compared to newer motors. Likewise, I would avoid the Mercury Verado, especially as finding good service may be a challenge - you don't want the additional complexity of a supercharger.

I would seriously consider an eTec 150 HO as a first choice, especially if your local person will be able to maintain it. The normally aspirated Merc 150 would also be good, as would the Yamaha F150.

Tim