Responses in red.
1) How reliable is the auto box? Very reliable
2) How reliable is the 4wd? Very reliable
3) Is it worth considering the 2.5 too or is 3.0 a much better engine? (2.5 has head gasket issues?) 3.0 is by far the best engine, had both and 2.5 is much more economical, 3.0 has more grunt but very smooth to drive.
4) Are there any known issues, such as electric windows fail after 80k miles or air-con tends to fail because of "x" part needing replacing? I had to recharge the aircon (£30) on my 3.0 at 9 years old so not a deal breaker.
5) Do they rust? Not unless damage is left untreated, like any motor.
6) What's parts availability like? Can you go to the local motor factors or is it dealer only? Many parts are dealer only but the main service items are easily avialable from third parties. They do not however break down often.
7) Can these be DIY serviced (oil, filters, gearbox and diff oil) easily with a jack, axle stands and a socket set? I am not mechanically gifted but routine maintenance is simple to do. Lots of good videos are available on You Tube.
8) I want DAB and Bluetooth so I will need to change the headunit. Is this easy to do? For DAB yes unless you use a separate DAB radio and connect by wire. My 2.5 had bluetooth for telephone installed.
9) What's the real world MPG? With the 3.0 I could get 35/36 on a careful long run but generally in the range 28-32. With the 2.5 I could get 42 on a careful long run but generally 32-36.
10) The 3.0 is a chain-driven engine isn't it? What about the 2.5? The 3.0 is chain driven but I believe the 2.5 is not, however I am not 100% sure.
Hope this helps.
ft
Just to add I have had BMW straight 6, Saab v6s and the Subaru H6 knocks them all for smoothness and power delivery