Simple - All Outbacks came from the factory with EJ25 except 1996 manuals - they came with EJ22's.
1996-1999 offerings:
Legacy GT's, LSi, and SUS came with EJ25.
Legacy brighton, L, and LS came with EJ22.
EJ25 unless someone swapped it.
EJ22's are one of Subaru's best motors. No headgasket issues in 1990-1998 EJ22's or EJ18's. If you don't overheat them or run them low oil and properly maintain them - they will run as long as you care to maintain the rest of the vehicle. 300,000 miles is rather easy...at this point the only issue you run into is prior maintenance/history - after 15 years a lot of things can happen. Get a nice one-owner EJ22 and maintain it and you got a good shot at 300,000 reliable miles from the engine. My EJ18 swapped LSi is getting in to 200,000 miles and 300,000 is almost a no brainer from the engine if the vehicle can make it that long (3 deer wrecks and it was totaled earlier this year at 75+ mph on the interstate by some friends...but i've rebuilt it every time).
1996 and earlier are non-interference.
1997 and up are interference (though I have seen one report of a 1996 being interference, so there may be some bleed over either way).
First step when buying any older used Subaru is to replace the complete timing gear - pulleys, tensioner, and belt. The newer style tensioners (around 1997 and up) are less reliable and should be replaced. The old style almost never fail and you can leave them. But you should still replace all the pulleys, after 15 years they are lacking grease and can heat, seize, and fail or break the belt (even if it's new). Ebay is the best place - older style kits are $60-$100 and newer style tensioner kits are $160-$200 (more expensive for a less reliable part, BOO!).
All 1990-1998 EJ22's are essentially the same engine with minor changes and different stuff bolted to them and thus are equal in reliability. Non interference is preferred by purest so a timing belt failure means no engine repair costs...though properly maintained timing belts rarely fail.
I've never seen any EJ22's have headgasket issues but I have heard of folks say the Phase II EJ22 (1999-2001) can. It's a very rare engine to begin with as they were phasing those out so you don't see them much. The bore being smaller it's hard for me to imagine that it's just as bad as the EJ25.
