Like all performance cars, you need one thats been looked after. Problem with the impreza's, you get people people buying them that cant afford to fix them properly when the go wrong, and you end up with a car that has been bodged half its life.
Your probably better looking for a bog standard UK example thats been serviced at Suabru all its life, and not been messed with. They come up every now and again, but sell quickly too.
If your handy with cars and dont mind repairing stuff yourself, they are not too bad to work on. If your no good with spanners etc, expect large labour costs, unless you can find a good reliable independant garage in your area.

something like this.

http://www.pistonheads.com/sales/3660896.htm