There's a very good X-Trail forum where you could get lots of good advice:
http://www.x-trail-uk.co.uk/
For what it's worth, we have a '56 plate 2.2 DCI. Last of the "old tech" mechanical diesels. Yes, on a cold startup they have been described as sounding like "someone kicking a canvas bag of spanners down a fire escape"! This is much improved by using a good quality fully synthetic 5W40 oil instead of the recommended 5W30.
I think the "oil cooler" referred to - on top of the engine, is actually the intercooler. They do seem to give trouble and it has been suggested that if ignored, can also knacker the turbo. Early cars (2003-4) were famous for turbo failures, but most should have had theirs replaced under warranty by now.
Up to early 2006, they were slightly more economical. Late 2006 saw the transition to "Euro 4" emissions levels and they got a bit thirstier. From 2007 onwards, they were face-lifted (referred to as the "T31" model rather than the "T30") and they were Renault-powered. More power, more refinement, more economy, but more complexity too.
On the whole, I have been a bit disappointed with ours. It has never "broken down" but in just under 70,000 miles it has needed a damper, 4 reconditioned brake calipers (for which they're famous!), an anti roll bar droplink, rear anti roll bar bushes, front lower arms...
On the plus side, the wife absolutely loves it (They are quite a nice drive in a lazy, torquey sort of way) and it's utterly brilliant in snow. Worth going for the Aventura spec with heated leather seats, Sat-nav, and Traction Control. The latter is important because it's not a "proper" 4x4 in that it doesn't have a set of low ratio gears or any diff locks, so the traction control uses the ABS pump to apply the brake to the spinning wheel(s).
