I've got a 2.0R 2006 Legacy Saloon.
I'm not that mechanically minded, but I'd certainly check under the wheel arches as there is a little lip that collects crap, could be a bit of a rust spot.
Oh and If it is a twin exhaust, have a look from the rear underneath to check the state of the exhaust Y pipe. I know mine needs replacing at some point, which is going to be a whole new exhaust, when i've got the spare cash.
Also, this thread is worth a read.
http://uk.subaruownersclub.com/forums/topic/1619-help-check-guide-buying-1st-subaru/