Re: Garden Office Skoolie
For the installation and amount of runtime you are talking about, I would definitely seek to :
1. get an 1800 RPM generator, not a 3600 RPM one
2. get a liquid-cooled engine over an air-cooled engine, and
3. have at least 2 CO detectors on board, so you don't take an endless nap while working with your computer.
I would be sure that the exhaust was ducted away. The RV stores have "Gen-turis," or something like that, a removable vertical stack that goes from the standard under-floor exhaust pipe and puts the exhaust above the roof. You might want to have a permanent stack from an "indoor" generator go up through the roof.
For fuel, I would be torn between diesel and propane. Propane does not spoil in long-term storage, but you will be burning enough fuel so that diesel spoilage would probably not be a problem. If you go with propane, you will need tanks with enough surface area on top of the liquid so it can boil into vapor as fast as you draw the vapor off. One barbecue-diameter tank probably would cause a medium-sized genny to sputter, but drawing from 3 or 4 tanks at once through a manifold works OK. I am responsible for a couple dozen home-sized generators, and when there is no horizontal "submarine" tank at a site, at least two 100 water-gallon tanks or five 100-pound tanks plumbed in parallel are standard.
Diesel is easier to come by and to handle, but is smelly, and can absorb water. The choice for me would likely be a toss-up based on what the bus ran on. For a gasser, I would probably install a propane genny, but with a diesel engine it is probably easier to plumb a diesel genny into the fuel system.
We have standardized on the 12 kW Onan quiet diesel auxiliary generators mentioned earlier to be mounted in the trucks at work. We have had no issues, except a dead genny start battery on one truck. By standardizing, we can do a "forklift repair" if need be, with the truck having twin gennys being a donor to the other truck with only one, or any future trucks we build. But I think these units are above your budget.
You probably have enough information to make a decision, you just have to find a unit at the right price with the most number of features that you want. A well-loved, substantially built used unit might serve you better than a brand-new throwaway model. You will find that you will want to make a sound-deadening compartment, and have it intake and exhaust cooling air through grilles to the outside. If you position the grilles and/or ducts to work with the machine's airflow, and prevent re-cycling the heated air, it will be happier.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.