My vote is for the inboard, and here's why:
You talk about motoring when it is flat and calm, or relatively speaking, and sailing the rest of the time. The reality is there are going to be those time that don't fit the plan. Okay, onward with my thoughts.
1. Transom design: The original transom was not designed to hang a heavy motor on it and you're going to have to structurally reinforce it big-time to hang all that weight on it. Crawling back in there to glass the reinforcing isn't going to be fun, and glassing on a vertical to keep it from sliding down requires some knowledge.
2. ICW: I've never heard of anyone who has only sailed the ICW, but then I live on the West Coast. Maybe someone has, but I've read a lot of posts here and almost everybody talks about motoring and not much sailing. Even if you're retired, tacking back and forth in a channel bucking a headwind or current with the hot sun beating down can get awfully old after a while. Skin cancer comes to mind. I'd rather be under the dodger (or east coast, bimini), out of the sun and with the remote control for the autopilot in my hand.
In the Northwest there are those that travel the inside passage which can be a half-mile to several miles wide, water depths in the 200 to 600+ feet, all the room in the world, little traffic, and they still motor!
If you keep the boat for a few years and use it a lot, my guess is the outboard will get awfully old.
3. Cavitation - and, swamping: Everybody talks about cavitation which is a definite problem, but what about swamping? Sure, the newer outboards are more enclosed than they used to be but are they watertight? What will the saltwater do to the outboard if it gets up inside the cowling? Corrode stuff? What does that do to the electrical components? Can it cause the motor to quit?
Plus, if you ever had to work on the outboard it'd be a pain. Just don't drop a tool or part - ker-plunk! Inboard is no problem.
4. Dingy: Why not use the outboard on a good dingy (with a 9.9 on a dingy you could really scoot) and shop around for an inboard replacement.
Sure an inboard will make noise but you'll be out in the cockpit anyhow so it'll be less. If there's no motor now that means you can do an easier job soundproofing (don't forget to provide an opening for air, though). If you re-engine I've seen used 1 and 2 cylinder diesels for sale where people are upsizing to a larger motor. Used engines are out there and diesel gets good mileage. Ya just gota beat the bushes.
5. House electrical: This was mentioned before but for the sake of supporting the inboard I'll mention it again. An inboard is good for charging batteries but an outboard is much less so. I'd also go solar. If you want anything cold on board you'll need some kind of refrigeration so generating electricity will be a must. An inboard will give you the options that the outboard can't.
I think you'll be much happier in the long run if you opted to go the inboard route.
By the way, is she the Puget Sound or So. Cal San Juan?
