I'd go with Onan. If nothing else, there's a much larger knowledge base about them here and on the tech issues forum, including a few Onan techs.
Hopefully the unit came from the factory "generator ready", with transfer switch, 120V power & 12V starter wiring, and maybe fuel line already installed. You'd need to buy the generator, hanger plates, start panel & maybe its wiring, and tailpipe (suggest adding an Onan resonator from the get-go). Onan is dam' proud of their stuff, and they price it accordingly, even the small items. I think this board collectively used to figure about $3,500-4000 for a new 2.5 or 2.8 installation including parts, but that's been several years ago.
Jim, "Happy as a carp in a septic tank."