All of the information in this thread is good.
FWIW, we have a 48kW Kohler that runs on propane. Maybe it's a bit big, but it runs the whole house (including appliances and central HVAC) without a problem. And the price difference was minimal between 30kW and 48kW.
Make sure you buy it through a dealer, and make sure the people who install it are certified by the manufacturer (it's usually the case for dealers).
We have a service contract for $250/year. They come out twice a year. One time is for a general inspection, the other time is for oil/filters/etc. More importantly, a service contract gives us priority in case something needs immediate fixing.