6k is a bit dear unless it is mint, 1999 was around the time they changed to the THOR engine which is definatley the better power unit.....it can be identified by the 'Banana Fingers' intake manifold.
Always check for any signs of cooling system issues, like traces of dried coolant in suspicious places.
Make sure she starts up with ease and no ominous blue smoke from the exhaust. Check for any tapping or ticking noises on startup (if left to stand for a while the tappets can leak back and take a few seconds to pump up again, but any startup ticking/tapping longer than 15-20 seconds needs investigating) also listen when she is warm and the oil has warmed up and thinned out.....top end is more than likely tappets, a lower down tapping/ticking could be a liner issue, although on the THOR engines it is rarer than the GEMS.
Contrary to popular belief, they shouldn't leak oil, at least not excessively. They should pull strong with no hesitation from initial acceleration, and the noise should be subdued and throaty (unless a SS or Cat -Back system has been fitted) Any exhaust rattles could be cats breaking down.
Check for the usual P38 things such as an eager EAS system, good looking air bladders, sniff the fuse box, check the Sport mode and Low Range function as they should. Steering should be direct, although if fitted with 18 or over rims, they can wander if the tyre pressures are anything other than 'just so'.
Just sit back and enjoy the V8 noise and grunt.....but 6k....unless it is a mint example or a nice looking 'Special Edition' model......I'd think of knocking him down or looking at others for a comparison......remember the Golden Rule with buying a Range Rover - Never ever buy a Range Rover with a fault no matter what the seller says, unless you understand and are prepared for the potential cost, time and heartache involved in repairing it.....