i had an isuzu pup with a 4 cylinder naturally aspirated diesel all thru college - once i blew my bug up! bought it brand new in 1988 for like $7900! we bought it in oregon to forgoe the smog equipment. it averaged 52 to the gallon. great for a ciommuting college student. they used to put those same motors in the troopers, until lots of power became "in style".
nissan xterras in central america come with 4 cylinder turbo diesels and have plenty of power, not to mention great mileage. trust me, we drove it like a rental!
not only cuz it was, but you have to to keep up with the costa ricans!!! i was impressed with how tough those xterras really are. it really took a beating. i believe that the h3's weigh quite a bit more than the xterras though and would probably require more push.
the defender 110 four door comes with an in line I-5 turbo diesel and gets over 20 mpg. not available in the US, only v-8. not sure on the dimensions of that motor, but i do think that the 110 four doors weigh just as much as the H3's.
as it was eluded to earier in this thread, it is unfortunate that the US views diesel as more dirty and pollutive than it actually is. diesel motors have become substantially more clean burning and quite in recent years. maybe it is just that i am going deaf?!?