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The SOHC EJ25's defining flaw is an external head-gasket leak: the gasket seeps oil and/or coolant at the head-to-block seam, classically once the car is well used (the corpus median on these complaints is ~86k miles, and it is a near-certainty by ~100k+ on an un-serviced example). It is the best-known Subaru issue of this era -- 237 federal complaints across the SOHC EJ25 models (Outback, Forester, Legacy, Impreza, MY2000-2011) name a failed head gasket, led by the Outback (120). It is usually a slow external seep rather than a sudden failure, but a correct repair -- heads off, resurfaced, timing components refreshed -- runs well into four figures, so it is a price-negotiation item more than a walk-away.

Check — Look for oily or crusty coolant residue where the heads meet the block, low or recently topped-off coolant, an oil sheen in the coolant or a milky film under the oil cap, and any overheating history. Ask for records of a prior head-gasket job and whether the timing belt, tensioner, and water pump were done with it.