I actually looked at a speed 6 when they first came out and I thought it drove great, shifted like butter. Not to sure how they are in the snow Or how reliable they are. I do know there is a lack of aftermarket support since they were only produced for two years but most engine mods are the same as the speed 3.
Back on topic to OP:
Try to give the car a cold start and check for any abnormal noises, smoke, ect.
Make sure to look underneath the best you can for any leaks, especially look at the axle boots to make sure there are no tears (very common).
If you have another person with you give the car a couple revs and check for smoke out of the exhaust to make sure it is not burning oil.
Obviously make sure the car does not grind any gears and that the clutch grabs nice and low.
Give the car some throttle and check in your reaar view mirror for any white smoke to make sure the turbo is in good shape.
If all checks out, take the car to a Subaru dealer or a mechanic you trust, have it looked over and do a compression and leakdown test.