I'm thinking Panasonic or Bosch mid-drive, maybe? I've had pretty good luck with Cyclone mid-drive units but I'm not so sure they would stand up to the kind of salt water and monsoon wet season abuse your likely to get in island habitation (I'm making some assumptions there of course and if incorrect please inform) and one of the two problems I've had with the Cyclone systems is that they are not 100% water proof sealed out of the box, although a tube of silicon calking does go a long way towards fixing that (the other problem I've had with them is that their smaller internal controller units burn up the weak internal controllers, but that's easily avoided by getting one of their models with an external controller).

Although I don't have any personal experience with the Panasonic and Bosch mid-drive units that come in the form of an integrated crank motor unit from what I've heard about them on this forum and the endless sphere forum they are among the best tightest sealed and fully integrated mid-drive units so they might be worth looking at for your needs. I personally could probably put together a Cyclone based unit that could handle your environment but it wouldn't come that way out of the box and I'd have to use quite a bit of shrink wrap, calking and some home fabricated weather proofing boxing and mounting options.

How does the islands infrastructure, regulations, and your wife feel about a trike or quad set-up? A lot more options available with that kind of configuration for weatherproofing since you don't have to maintain a narrow profile and could completely box up the drive system in a weather proof box with just an output shaft for a chain sprocket and a throttle and charging wire coming out of a weather proof sealed box.

I don't know what kind of a budget you have but one of the places I know of that would build you a bike using whatever set-up you wanted with custom weatherproofing options for your environment is Light Foot Cycles. I know about them because they are in my home state (no relation, I don't work for them and I build my own rather then pay their prices) and provided that you are willing to pay their prices they will build you just about anything you want customized to your specifications and unlike a lot of other custom bike builders they are very familiar with and build and deal in motor assisted bicycles (both electric and small efficient internal combustion). I've often used their photo pages for ideas for some of my own builds.