24v is much better for transmitting large amounts of power. If you will have a bow thruster,
electric windlass, etc. -- go with 24v. Providing 12v to a few things which are hard to find in 24v is a trivial problem.
Completely separate
engine (and generator)
charging and starting, with separate
alternator, and this can be 12v without any problem.
For AC power, 110/60 vs 230/50 depends on where you will spend the most time. Choose whichever system matches your most frequent cruising area.
Then, install an
inverter large enough to run all your AC
equipment (or a gang of them -- you can combine two or three Victron inverters). Separate
shore power inlet to run a separate
battery charger on the "other" voltage -- whichever one you did not choose for your native on board AC system.
So now when you are in a region which has power different from your "native" power, just use the second shore power inlet and run everything off the
inverter.
It will add complexity but also efficiency if besides the
battery charger, you can switch over your immersion
heater to the "other" voltage (to save inverter capacity), and have perhaps a few sockets with the "other" voltage for devices which don't care.
TIP: If you decide on 230/50 as your "native" AC power, size the
wiring so that the
boat can be converted to 110/60 later in case you end up living permanently somewhere with that power. That means the
wiring should have double the cross section of wiring which would be suitable for 230/50.