Hello & welcome, theres a buying guide here http://uk.subaruownersclub.com/forums/topic/1619-help-check-guide-buying-1st-subaru/#entry45963
It mainly covers petrol cars so I'd just pick bits out that don't apply, there have been issues with the glowplugs on the diesels if you have a search on here or google it I'm sure it will bring some stuff up, looks like some owners are having them replaced under warranty and some are having trouble,
I'd check to see if there has been anything done or in the history, unless it's a new car your looking at.
We're all quite friendly on here so no need to worry feel free to join in and ask as many questions as you want :-D