I did this with a friend helping on my car. All four seals plus the crank seal. Not fun. Mine did not leak yet but were dry and looked old.
Buy authentic Subaru seals and buy extras (I recommend six cam seals and two crank).
Figure out what you are going to use to remove the old seals. I wrecked a set of O-ring picks. Figure out what you are going to use to install the new seals. I tapped them in using the old seals, reversed, and a mallet. There are seal I install kits available though.
Yes it's a job and yes I would do it if you are keeping the car.