got to see the chocolate milkshake, in addition to the radiator level way down.
these engines don't start failing that way - ever. so you can almost guarantee the engine was run hot significantly, or for a long time with a failed gasket, or less likely - it's been previously (poorly) replaced.
that's what i was eluding too earlier when i said this current engine is likely suspect. i wouldn't have even considered repairing it, because it was highly unlikely to be a good engine, you're "fortunate" to see rare physical evidence of it. and you've already got the engine on the way, good luck dropping it in.
swapping an engine in that 2003 is not significantly different from any other Subaru engine swaps.
if you're the preventative type you can do the plugs and VCG's now while it's easy with the engine out and be done with it for 100,000 miles. the H6 plugs are robust and don't even show wear after 100,000 miles so they're not a huge concern but they should be replaced once in the life of a vehicle.
do the serpentine pulley bearings now - keep in mind those are 60,000 mile replacement items due to their high failure rates on H6 engines. they're super easy to do though - remove the two pulleys and tap out and back in new bearings. standard bearings and they tap out really easy. i get japanese quality stuff off amazon, they're widely available and local stores will have them too. replace them again in 60k or remove and check at least. they fail at wildly varying mileages, so there's really no good mileage or reason to "60,000" miles but it's early enough to catch most failures. you should just have some consistent mileage in mind.
i've even bought JDM engines that had failed bearings in them - i assume they had to be junked for that reason.
compared to what you current have - an engine that's nearly guaranteed to have been run hot and with extant headgasket issues,