In the price range that I think you are looking at, I would go with Generac.

Gas/diesel, as you mentioned, are spendy and a lot more maintenance intensive. For what you want, especially with propane on site, I would go propane.

I would just work with Generac and let them know what you want/need. The last set of generators I did for a customer ended up being from Cummins with their ATS, but Generac was very helpful in the quoting process. (They were slightly bigger than you're looking for - 80KW gensets).