There are two types of marine
generators: direct drive and belt driven.
The direct drive types have to run the engine
at an rpm that matches the output frequency: typically 3,600 or 1,800 rpm for 60 hz and 3,000 or 1,500 for 50 hz. You can either have a slow engine- Kohler, Northern Lights
, etc or a fast engine- Fischer Panda
with a direct drive. The slow engine
gensets at the same KW output are a whole lot heavier.
The belt drive types can run the engine at any practical rpm but most such as NextGen and the small Onans run the engine at about 2,800/2,900 rpm for 60 hz and about 2,400 rpm for 50 hz.
It sounds like you need 50 hz power as you noted that the Kohler runs at 1,500 rpm.
1,500rpm for a marine generator
is pretty slow and it requires a big chunk of iron to make enough power at that rpm to run the genset end.
Almost all 1,500/1,800 rpm marine gensets in the 5 kw range have three cylinders and run pretty quiet.
The NextGen/Onan gensets at 2,400 rpm and 5 KW are 2 cylinders and less cylinders plus more rpm means that they are noisier. But they are a whole lot lighter, almost half the weight of the slow rpm gensets.
The slower the genset runs all things being equal, the longer it will last.
So in summary: slow means quiet, heavy and long lived. Faster (either the direct drive types or the belt drive types) means noisier, less long lived and lighter.
It all depends on what you want, what room you have to install it, whether weight is important, etc.