i just got rid of my 07 wrx hatch 2.5,head gasket failed.early engine are terrible 07 but i was told round 09 10 are much better,a 07 engine if you plan to boot it about then expect a bill at some point or get the engine built,or just drive it like a granny and it sould be ok,changing oil i would say dont really make a diffrence,i changed my oil every 5000 mile in the hatch used shell full synthetic and genuine subaru filter and it did not stop it failing.at the end of the day any engine head gasket can fail,dont matter if its a 2.0 or a 2.5 if its going to go it will,you buy a second hand car,may have full service history but that dont tell you how its been driven,so you buy a car on your judgement and if it fails it fails its just 1 them things.even though the head gasket failed on my hatch i am still thinking of getting another 1 just a later model maybe a 09.