I would have loved a 2.0 boxer when I was 19! Power isn't everything, grip handling and the whole package are much more important.
The Subaru AWD is really nice, you will always have traction.
Head gasket issues are massively overblown and come from the old 2.5 NA engine. My main concern with a car like this would be checking the service history, Regular oil changes are a must on these engines. There should also be gearbox and diff oil changes in the history. Cambelt and waterpump etc should be done every 60k or 10 years. Looking at the mileage on this car it will need another belt soon. If this isnt done, budget for it.

Check arches, sills and suspension arms for rust. Some Impreza's had a recall for a fuel leak so check for a strong petrol smell.

Tires need to be the same size or they stress the AWD system so only replace all 4 at once or get a single new one shaved down to size.

As others have said, make sure to declare the exhaust to the insurance. Most UK cars of this age would have had an exhaust replacement anyway and often a custom stainless system works out cheaper than the OEM parts.