[LizardOfBodom]

Hi
most related questions will be answered by D5 users here, in general its the best diesel engine in P1 platform (edit: pre-FL model I meant, looking at T5R90210's post) you can get and from what I heard, wont dissapoint you with its reliability nor performance. MUCH better option than PSA diesels, thats for sure.
M66 6speed gearbox is a good piece of technology, have it on my T5 aswell and its not something you should have problems with (*I THINK*) - robust and fit-for-purpose, AT gearboxes may cause more issues from what I heard. My has 143k miles on the clock and gearbox is the least of my worries. Clutch and DMFs on other hand, thats something you will need to invest in , if not done. huge torque on those D5's.
As for aux in/usb - be careful, only FL models have them (2007/8+). Older cars like mine have none of it and no physical possiblity to add it later (unless 600e+ spent on dedicated unit).
In general with those cars - watch out for sunroof, if it has it - its 99% possibility it will give you problems (search "sunroof drain issue" ) and also try to find model with xenon headlamps. Stock halogens are rubbish and also prone to mist/flake etc so in general that is a achilles feet of our cars.
Also, as those cars are very electronically-complicated, make sure you test all components and if possible - get it tested in volvo or with someone who has vida+dice diagnostic unit, as any odd hidden electrical gremlins will cause you grief later on.
best of luck in car hunt!