I off road a disco, its a 200TDi.
The 300TDi is better and produces more power. The 200TDi can be brought up to same by bigger intercooler and tweaking pump.
Things to look out for:
Rust on chassis or rear cross member, Rusty boot floor, rusty front floors. These can be welded but welding is expensive.
The body is alloy so should be Ok, the roof is metal though and prone to odd spots of rust if had a roof rack on.
Everthing else is bolt on bolt off and can be replaced reasonably cheaply.
They have a good return on MPG, but are unreliable if not looked after. All parts are readily avilable off shelf as are bits for modification or improvements.
They are quite noisey (I think) compared to the Nissans, my wife thinks my old Patrol was noisier though, so opinions do vary.
On road they do tend to rock and roll a bit but the 300 TDi is better than the 200TDi. They are still a bit agricultural compared to the refinement of a Nissan, but compared to a Land rover they are warm and comfortable.
Off road they come into there own, and even as a standard vehicle outperform the Nissans.
As with all vehicles you get bad, good and horrible ones, you pays yer money and takes your chances.
If the vehicle is structurly Ok the rest can be fixed. A complete engine for example is only about £400 - £500 second hand.