I have a soft spot for the Onan 30kw generators with the Ford 240 straight six. Propane I like the best. They are 20kw when wired single phase. Engine parts are cheap. If you find one with the older pre late 70s controls, they are all analog, and the manual diagrams tells you all the components and their values.
They are 1800rpm and don't sound that loud even with no muffler. Propane makes a lot smoother sound than gasoline. Fuel system parts are easy to get, at least for propane and gasoline. The difference between the propane and NG fuel systems is a spring in one of the regulators - no spring for propane. Gaseous fuels are so much nicer to deal with since they don't go bad.
The last guy I know who found one payed $500, and that was last year. I'm sure there is more than $500 of copper inside that thing. He got the deal of the decade. It came with a trailer also. There are different versions, some for indoors and some outdoors. His had sat outdoors, and it was an indoors version, but it still ran fine. No water damage to anything important, just maybe ten years of faded paint. Once the mud daubers nest got cleaned out of the propane filter screen, it started right up.
If I needed one, I would go for the older version. Cummins now owns Onan and maybe think customers are made of gold. There is little to go wrong other than the electronics in the newer versions. It's the same industrial engine that is in everything ftom wood chippers to welders to airport tugs and sawmills. Points ignition. Basic alternator. Little to go wrong.
Can you tell I like these things?
That being said, they are probably twice as big as a generator with a 3600rpm small engine. They also weigh about 1500lbs.