Congratulations on your low mileage find!
You may already be thinking of the things that should be looked at on an older low mileage find like this, but I will list some stuff for your consideration anyway.
Everything that is rubber is probably 18 years old...and probably needs replacing. That would include the tires (manufacture date is on the sidewall), the timing belt (my manual says replace after x miles OR x years), windshield wiper blades, probably coolant hoses, etc.
Definitely do an oil change, flush the old brake fluid out of the lines (hygroscopic), and change the coolant. I would probably change all the fluids.
I hope you get many years service out of your Subaru!