There is lots of good stuff to learn in S/V Jedi's AC thread.
Also ask yourself this question...do you really need AC, how much inverter
power do you need? That is, aside from being able to charge with the inverter-charger now & then. If you truly don't plan to be at a dock
very often it's something to think about. Of course if you have air conditioning
...then there's not many options.
Note S/V Jedi's load-sharing Victron inverter-charger, I can 2nd good experience with those, the BMV602 monitors, Orion DC-DC converters, etc.
Re 12V vs 24V, add up all your 12V loads (the lights, etc.) and look at powering them with the Orion DC-DC converter. All the big loads (and charging) should be 24V and you'll be gaining some efficiency. Every bit of which adds up when you're producing your own energy offshore
Use as much inverter-charger power (more than one can be paralleled) powered from the Genset as you can afford (within reason), and that the Genset can handle. Also, your engine
will have no problem driving two large 160A x 28V alternators, though be sure to be able to turn them off (using switches in series to the on/off ign input on the external regulators).
However, how you mount the dual alternators is a whole other discussion...with belt and friction losses you could draw in the neighborhood of 20hp (10hp each) from the engine at full output. That's a lot, and makes the design of the layout important (and possibly expensive).
But being able to get 300+A @ 28V from your engine could save you lots of charging
hours and possibly eliminate the need for the Genset altogether (bringing you back to the "how much AC?" question).