Bear in mind that if you start exchanging engines between the 300tdi vehicle and the Isuzu vehicle, you'll have to get at least the vehicle that ends up with the Isuzu engine certified by an engineer, and probably the 300tdi recipient too, seeing as how you'll be welding in new engine mounts and all. That's going to add at least another $800 per engineer cert to the final bill.
One option worth considering is contacting TRS in Adelaide, as they are currently wrecking three Isuzu Countys. I enquired a few weeks ago, and they were selling off engine, radiator, bellhousing, and all the bits form under the engine for $3000, then you just have to source your choice of gearbox and transfer case (from TRS as well, if you want), and you're set.
