My 02 wagon has 194k on it, I'm the 4th owner and I know it was driven like it was stolen. Took a chance when I bought it not running (no tranny installed) and other than the prior "build" (someone slapped the engine together) being complete garbage, I'm just doing basic maintenance like suspension, bushings, bearings (194k and still has 2 original wheel bearings), etc it's still going strong.
Between corrosion and other mods/maintenance you shouldn't really need to worry about it. Buy from Subaru if you can and get their OEM extended warranty if possible, but only if it's the Subaru warranty as others commonly rip people off when things need fixing. Also when you buy used, set aside enough money to replace the motor or tranny just in case...even if those things don't go out soon after, cost of maintenance can be high depending on these variables. Oh and test drive or bust, 5speed you should check the clutch in 5th at WOT to make sure it still holds.
The biggest issue people talk about with the EJ255s and 5speeds are the emissions bs and the gearbox, as they can be broken if you bang through the gears and drive it like a truck. Whether you are at stock power levels or making 700whp, it's all how you drive it!