I can see You are on Land Rover UK Forum too, so many of the subject´s is known to You.
1. The biggeste problem to overlanding is the fuel pump in the tank. Take a spare and fuel filters of course. The pump dont like dirt and can lose pressure gradually.
2. Oil in the engine harness and in ECU, problem up VIN XA173796, but later Vin have been reported. Gives misfire, porely performance. Solution (if no problem today) is to change fuel injector harness every 3 year (not so expensive)
3. Make sure the ECU is upgraded with the most new software(no new release in several years)
4. Flywheel disintegrate between VIN XA145775 to XA181042
5. Very early td5 had plastic dowles in the head. Must be changed for steel ones.
6. The dreaded oil pump bolt problem. Nobody knows how many engines are effected, but for peace in mind take off the engine oilsump and take off the bolt and tighten again with Loctite 243.
7. Fuelregulator can stick. Can also leak fuel. Carry a gasket and inspect.
8. On early VIN, the watersensor in the fuelfilter can be unreliable. Newer ones haven´t a sensor. Take it of and drain filter at service.
9. A td5 must newer use water. Leaks can be difficult to find. Leak can be in hoses, waterpumpgasket or worst, a small crack in the head with local overheating which can´t be seen on the tempgauge.
10. The wastegate vacuum "something" can stick (needs motion and WD40).
11. Studbolts in exhaustmanifold can shear off if getting cold suddenly (some people say so).
Look af http://www.web-rover.co.uk/nav.php?p=home
for more information.
Many problems is relatede to early VIN and only the fuelpump and the oil in harness is common, I think.
Carry a sensor for the flywheel in spare, the only sensor that can stop the viehcle, but others sensor such as MAF could be taken as spare.
If You take another engine and list possible known problems, I think there will be list too.
And all problems don´t come at the same time if ever.