The best idea is to mod with a purpose. If your purpose is to eventually do a low boost turbo system then mod with things that will maybe give you a slight power or handling increase but will still be able to be used once the turbo goes on.
Suggested Mods w\ future turbo in mind:
Catback Exhaust (2.5" or larger if turbo is in the future)
Up-Rated Clutch (Stock Clutches begin to slip pretty well under stock power much less turbo power)
Lightened Flywheel (Lightened Flywheels reduce the rotating mass of the car allowing it to rev quicker. When weight reduction is talked about reducing rotating weight is about 3 time more noticable then dead weight. You will definitely feel more power here, I would venture to say you would even see a gas mileage improvement, however you will need to learn to slip the clutch a little more during launch)
Underdrive/Lightened Pulleys (These don't make as big of a difference as clutch/flywheel improvements but will again reduce rotating mass and look nice as well)
Ebay Intake (Even though you wouldn't be able to keep the NA intake once a turbo system is installed, it would be worth paying $100 for the improvement in power and looks while inbetween. You shouldn't have any trouble selling this once you do turbo upgrade)
One of the biggest pointers I could give you when modding your subaru: Keep your stock parts on hand. Do not sell these. If you regret any mod that you do and even think about swapping the stock parts back on you will pay an arm and a leg from a dealer. As well you may be able to pick up used parts on these boards but you really never know what your going to get.
Take care of suspension and braking before upgrading power. Our fat Legacy family cars can really be transformed with stiffer springs and strut. Adjustable rear swaybar is another cheap easy mod.