Long term in less developed countries, I would go 300tdi every time, as working on Land Rovers every day I see TD5s in all their unreliable detail. The 2.8 tgv engine is better than either, but make sure you've got a GOOD/recon/new gearbox (or auto) to take the torque, and I would advise a 4 pinion front differential and heavy duty (Ashcroft) halfshafts and cv joints (although for peace of mind I'd have those whatever engine and indeed do in my 300 110)
If it has to be a TD5, go with a new clutch AND flywheel as the dual mass flywheel is a tricky one to source and expensive to ship when you're abroad! Get the head gasket changed and steel head locating dowels fitted in the block (sorry Mr Land Rover, but plastic? What were you thinking?) Might also have the sump removed to check the oil pump drive gear is fitted correctly. Take at least 1 spare injector harness, and get the fuel pressure regulator block replaced so it doesn't drip diesel onto the starter and kill it. (If there's evidence of leakage already, I'd add a QUALITY starter motor to the spares kit too, or fit it before departure.)
Might be worth carrying a spare ecu (tested on the car before departure!) in case you don't spot the oil contamination coming down the injector harness in time.
A spare in tank fuel pump might be handy as they don't like poor quality fuel. At least they're easier to change in a 110 than a 90.
Make sure there's no evidence of water pump leakage (difficult to spot, ask Nene) and finally (for now!) if you're getting a performance upgrade (they do go well I have to say) get the exhaust manifold studs uprated to 10mm otherwise they fracture and the manifold warps...
I've just done 6500 miles through Europe and Morocco, last month, in a rebuilt 1983 110 with 300tdi fitted, and my only real gripes are the oil leaks from diff, transfer box and steering box, dust coming in through every orifice and non genuine front shocker bushes collapsing after 1 day en piste. (Amazing what old conveyor belt rubber can be used for - started out as mudflaps!) I'm sure a TLC would not have had these problems, but that's another thread;-)
If that doesn't tell you my preference... Bear in mind a 300tdi only requires 12 volts to the fuel solenoid to run. A TD5 needs all sorts of electrical impulses to be correct or it will hardly tick over let alone drive you to the nearest DHL office!
As far as gearboxes, Land Rover auto's are reasonably reliable if serviced properly-it's down to personal preference, and the tow start if it won't go argument for manuals is a small thing to weigh against knee fatigue in heavy traffic on the Kathmandu ringroad with the heavy duty clutch on the 2.8 engine!
Safe travels whichever your choice, most vehicle problems are surmountable in the end.
Simon.