Private and dealer both have upside and downside issues. It seems a of people find this hard (remembering that I've helped a ton of people buy subarus), but it seems easy to me. Every car I've bought from someone who clearly had a reason to sell - retired, moving, mobile people like military/grad/medical, car needed a new fender, family of 6 (the new baby can't possibly fit in their current car)....has been a great vehicle. Every car where the person didn't have a real reason to sell (they always have "a" reason - company car, needed something new....lots of 'artifical' or convenient 'reasons' to sell that are described to a prospective buyer but they aren't real reasons) - is when you are likely into a problematic vehicle.
Dealers have some upsides too, usually on newer vehicles. A dealership purchase rarely has any upside to $5k vehicles - they get a car at auction they know nothing about that's had 3 prior owners and know known recent or past history. In my mind it's almost stupid to only look at dealers for sub $10k cars - you're guaranteeing to know nothing of the past history/ownership that way. Of course look there as well, but also consider private. I'll fly far away for a great private vehicle over a dealer. You almost always get a "worse" vehicle from a dealer of older, lower cost Subaru's because the nature of auctions/trade-ins is such that you're guaranteed to not know anything about the car. Unless yo'ure in a small or slow market where the trade-ins are local and known, but that's not the norm.
You won't find one that replaces headgaskets on H6's - they're too costly and labor prohibitive. And I wouldn't trust that it's done right unless I know who did it and that OEM gaskets were used and the heads were resurfaced.