Anything I can do to help a Kiwi I'm at the front of the line. (I was over there 3 years ago racing-stayed in Napier-absoulutly LOVED it!)
240k on an unmaintained gas motor is alot--on something that has had normal service is on the hi side--but not terrible. On a diesel 240k is nothing at all.
My diesel 16 pass. shuttle bus got 11 mpg on its maiden voyage--but that was in very hilly country-i'm sure it would do better on flat ground. I have driven (gas) large ford cargo vans that have gotten around 12mpg-so yes its believible.
I've helpped out traveling kiwis before--i assume you plan on buying it-traveling-then selling it off when your done--correct? A diesel has more value at resale.
Diesel fuel around here is now running about 20 cents a gallon more than gas.
So its close to even--I'd go with the diesel with the hope of getting more $ at the end of the trip.
If theres any more help I can be (I'm 1/2 way in between NY + Boston) you can contact me at firstname.lastname@example.org
You owe me a pie!