Andy R, don't overthink the running gear. Only the last four ships in the class had twin rudders (experimental for the ensuing Gearing class I'm told) A single rudder is easier to setup for RC, and in my experience on the water, allows the ship to handle very well indeed!
Motor to 2mm shaft couplings. I've been using some fine bore model diesel fuel tube. It just works
Motors? Mabuchi 280 x 2 bolted to the kit's motor plate mount. Performance is MORE than adequate!
Battery 4 cell sub C 5000mah (4.8v nominal rating) I have no honest idea how long the runtime is because it lasts that long.
One last tip. DO NOT MAKE THE DECK REMOVABLE! Fix and seal the deck to the hull and make the deck houses and superstructures demountable.
Doing this has allowed me to sail my Fletcher in quite extreme weather, whilst only shipping half a teaspoonful of water.
Those are my recommendations based on experience of sailing a Lindberg Fletcher.
In fact I got commissioned to build another 'sailer' on the basis of my initial build(!)