Can second the recommendation about Gary Elliott. He has some really spankingly good looking cars and is only in the business for fun...... He's already very successful in other areas.
If only doing 5k a year, go for a petrol. The 2006 onwards jag 4.4 is a good and much sought after engine. But it doesn't like LPG, unlike the earlier 4.4 BMW which is excellent. Also the cost is much cheaper than a diesel so the extra cost of fuel is minimal. You need to be doing over 10-15,000 miles a year to make a diesel worthwhile.
A diesel doesn't like short trips and needs to do high revs and long trips from time to time to clear the particulate filter (expensive to replace cats etc) and the turbos have been known to go, along with the water cooled alternators. Expensive repairs!!
Personally I would buy a supercharged 4.2 litre (or if budget fits a 5.0L which gets better consumption and engine than the 4.2) and sit back and enjoy. These variants also get all the toys such as rear seat entertainment (RSE), upgraded sound system etc.
In terms of buying, I would recommend buying from a forum member as you can trace the probs/successes of the vehicle. I bought my 160,000 miler from a forum member sight unseen as I know his reputation and knowledge care etc had been superb. And it was a good decision.
Ping me on a private email if you want any more individual advice. Glad to help. I was in your situation 5 years ago and won't have any other car now!