Scout 80 and Scout 800 rigs are where the real investment value in these rigs is right now in my opinion, interest is greatly increasing in 'em and not from IH enthusiasts, but from collectors of old iron in general.
And if the rig you can access is truly set up as a fire dept. Rig, then it May be even more special, depending upon how hacked it is.
Scout II units are a dime a dozen, but decent Scout 80/800 are disappearing from the marketplace rapidly.
Fuel mileage??? Regarding any IH product??? Is this a joke??? No IH vehicle will get any kind of "mileage" if you are comparing to a four cylinder import built from the mid-70's on. These motors will struggle for maybe 15mpg due to extreme vehicle weight, stoopid transmission gearing, and the fact these are truck/industrial engines with no comparison between toyhondasans!
All the same issues with rot, long-term inactivity, etc. Pertain to s80/800 as well as any IH light line vehicle. The Scout 80 was very crudely engineered using many off-the-shelf components, and as such you simply have to understand going in they were not engineered/built to even the level of the Scout II, which have their own design issues, particularly pertaining to poor body system drainage and rot. They are what they are, you can easily improve upon some aspects, but overall they are a vehicle frozen in time to was brought to market very quickly back in the day!