Most of the small diesel
engines used in the size generators you are talking about are incredibly reliable machines if installed properly and well cared for. Almost all the problems generators have come from the electrical
side. And most of those problems come from poor cooling
and/or salt water
Most AC generators do NOT use stock AC alternators, some are not even really alternator based, but are actually armature based units. They are brand specific including the voltage control.
There are two different kinds, the fixed speed where the rotation speed of the engine is fixed and sets the frequency of the power
(usually 1800 RPM
for 60 Hz) and variable speed units which convert variable voltage and frequency output to the desired power
specification. There are pluses and minuses to both.
ALMOST all will be air cooled, although you can find some permanent magnet types that are water
cooled. WhisperPower makes those, and I am sure there are others.
So much of the choice really depends on how you will use it, and how often, how much space you have to install it, and how clean and dry the space is. Are you weight sensitive? And who has part availability where you will be cruising.