I am not sure I would purchase a bike, sight unseen, especially a Brompton. You can't test the hinges to see if they are worn, you don't know how much juice the battery can hold and what consumables need replacing. Purchasing in person would be the better option.

Rust is an issue but with care that can be mitigated. I would go for the C Line as it is less expensive. Sure the P Line has some titanium parts but you still have to care for everything else. The C Line has two more parts to care for. Heck, even a T line would need care. The only beneficial thing the P Line will be the lighter weight to help when you carry the bike on and off your boat.

Common in these boards is "more gears are better" mentality. I do not follow this mantra. Three speeds are more than enough and if you have electric assist, does it really matter? Walking up a hill isn't a crime in the cycling world.

My neighbour has an electric Brompton and she swears by this bike, she just loves it. I haven't seen what version she has but she rarely touches her non-electrified Brompton anymore.

Good luck with your purchase, your plan sound excellent and well thought out.