Subaru dealerships sell a coolant conditioner that is required after a hg replacement to keep a warranty intact.
that conditioner will NOT prevent the typical internal head gasket leak, exhaust gasses into the coolant, that these engine usually have.
it could well be piston slap.
there is a test for that, in addition too ''does it get quieter when warm?''
pull the plug wires one at a time, and see which cylinder is making the noise.
if that does not locate the noise, it is not piston slap or rod knock.
in which case it could be the timing belt tensioner making noise.
if it is rod knock, it will not last very long.
so i would drive it until it fails and then replace it, the engine that is.
you CANNOT replace the rod or crank bearings without splitting the crank case.
and usually it is easier, cheaper, to replace the whole engine than to split the case.
i bought a 97 OBW with engine noise.
i drove it home expecting to swap in an ej22 which i already had.
while i was driving it to determine if it was a ''wrist pin'' or what, i learned about piston slap.
i drove the car for 40k miles and it never got worse or failed.
and i sold it for more than i paid for it.
plus, any money, or parts you put into this engine you will be able to use on a replacement engine.
the ej25 timing belt components, (water pump, 3 idlers, and tensioner) are all usable on the ej22.
(the tensioner is year specific.)
so if the engine fails next year, you will only be out the price of the belt.
buy the car.
but do not assume that because you know cars, that you will KNOW suabru engines.
there are oddities you need to be aware of.
do your reading and learn.
look for posts on
timing belt swap
ej22 engine swap
''i just installed a new timing belt and now it will not start?''
good luck and enjoy your new car.