Amongst other buying guides on here, this is an interesting read.
http://www.pistonheads.com/features/ph-buying-guides/ph-buying-guide-range-rover-l322/27046
Gearboxes are a large weak spot and often need a rebuild, you need to buy carefully. They are complicated cars with many many things to go wrong, they are also much harder to fix for the DIY mechanic than a P38. Most people would advise a large war chest is made available in order to cover the inevitable bills!
Something to carefully consider, if a vehicle is on a 55 plate then it will be £290 a year to tax where as if its on an 06 then it will more than likely be £505 a year as its in the new higher brackets which came into force after 23rd March 06.
I'm sure others would disagree but I would expect the TD6 to be fairly lethargic (although perhaps not to bad compared to a TD5?) You might find a V8 gives you a far better car for the money, especially if you can find the Jaguar engined 4.4 V8 which came in during 2005 I think. I am told they have a 6 speed gearbox which further improves performance and reduces fuel consumption. Alternatively late 2006 I think the 3.0 diesel was finally replaced with the 3.6 TD V8 which gives reasonable power and reasonable economy but appears to have issues with turbocharger failure. But this could be owner related, thrashing from cold and not allowing to cool before switching of etc.