If you're going to swap in a diesel, i'd use a 300Tdi. very similar in layout to the 2.5 NA, and the block of the 300 is a descendant of the 2.5, so it has the bulletproofness of it.
The military use the 300Tdi in their current "wolf" XD rovers, and it's a very good engine. as much electrical stuff on it as the 2.5 (glowplugs,starter,alternator,fuel solenoid and water temp sender). Later 300's had EGR (Exhaust Gas Recycling) valves on the exhaust manifold, with an ECU to control it. this can all be junked and replaced with factory non EGR parts (it's only a blanking plate oyu need for the exhaust manifold anyway)
111bhp as opposed to 67 of the 2.5NA. can be tweeked easily (turbo boost and fuelling) to give around 130 RELIABLE horsepower.
the 300 can be mated to the LT77 gearbox that the 2.5 usually bolted to, so no major issues there, unless you fancied the newer R380.