I know that you can use the stock Forester suspension on the WRX. The only down side is it will raise the WRX about 1".
You need to check the spring rates to make sure that they will work for you. Also, check the travel of the coilovers so if you go low it will not bind. Check the mileage and make sure they are working and not blown. Check on the cost an ease of a rebuild, some have to be sent overseas to get rebuilt.
Buy coilover is never quick and easy. I would pass even though it looks like a great deal.