Personally, I would not buy a car whose service records were not available. And, if the car is Turbo-equipped, the risk is even greater, due to the need for more frequent oil changes, and in some cases the need for special motor oil with those engines.
Since Subaru has recently changed their oil change interval for Turbo-equipped engines to 3,500 miles (the "Severe Service" maintenance schedule), it would be wise for you to adhere to that schedule, even if the maintenance schedule that came with your car allows for less frequent oil changes.
The problem is that the oil in Turbo-equipped engines tends to "coke" as a result of the extreme heat generated in the Turbocharger. This "coked" oil damages bearings in the Turbocharger and is then circulated through the engine where it damages the main bearings. Hopefully that damage has not already taken place, but since it appears that your engine is using oil, it is possible that this scenario is being played out.
My thoughts are to take the car to the dealership, as long as you have faith in them, and see what they tell you. After their assessment, you will be better able to make a decision regarding repair, replacement, or whatever.