Beamishken's suggestion is good about 5hp as good all-round auxilliary power, given the sort of distances you're doing.
In addition - think about installing stainless steel U bolt on transom knee. These outboards weren't designed for the kind of stresses they'll experience as auxilliary engine, especially on saddle area, so having the engine tethered in the up position with a ratchet strap provides additional tension on it to stop it vibrating when underway.
I flush my Mariner 4hp with fresh water too every trip, regardless of whether I've used it. Salt gets everwhere, and the tell-tales on these small outboards are prone to blocking. Nothing a piece of strimmer nylon can't sort!