I agree with your choice of the Beta Marine engine.
Note that many (most? /all?) Beta Marine Engines are "marin-ized" Kubota Diesels.
You may not know that the Universal 5416 shares some history with the Beta engine;
In 1976 the first Universal diesel was introduced. Designated the Model 5416, it was a 45 cu in, 16 hp, 2 cylinder, fresh-water-cooled engine producing 16 hp @ ? rpm (intermittent), based on the Kubota Z-751 block and fitted with a Hurth HBW-50 marine transmission. It was built from 1976 to 1982. Approximately 1,000 were sold.
- From History of the Universal Motor Company
I will also chime in that the higher amperage alternator, and the serpentine belt, are no-brain decisions if you plan to cruise.
[EDIT]: I would look through their options to find if an Oil Pressure gauge can be added to either panel. If not, I would get the panel with the volt meter, and swap it for an oil pressure gauge. IMHO, understanding what the oil pressure reading is at any moment, and correlating this to RPM and temp, is more
important than knowing the voltage of your electrical system.