Guess its up to you whether you can justify the $55k+ for a new one.
There is a 2004 td5 one for sale on ebay here with 77k km for $31k
2004 Land Rover Defender 130 (4X4) White 5 Speed M - eBay, Passenger Vehicles, Cars, Cars, Bikes, Boats. (end time 17-Aug-08 12:02:14 AEST)
With regard to longevity of the TD5, I am not certain, but have read of them up to 300k km already. I suppose as long as you look after it and service it properly, I can see it lasting quite a long time.
 I think the TD5 is a 'stronger' engine than the puma, at least in terms of potential output figures, which would translate into it being mechanically stronger to an extent.
The new one is going to be easier to drive with a nicer gearbox, earlier torque delivery, better dash, and better air con, but that is then counteracted by the extra complciation, at least with the dash and Hevac, which does concern me a bit if you were say to accidentally sink it in a river.
 I also think the Puma engines ones are due to get stronger front shafts and Cv's shortly due to them being able to break them during engine braking!
 
A new one as you say would be nicely run in when you left for the tour and you would know its entire history.
One positive of a second hand one is that it is likely to have some of the extras already fitted which may save you even more.
Rgds
Pete
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