OK, the RS is N/A right?
Would go for the lowest mileage one you can. 100,000 km is no problem at all for an Impreza engine (turbo at least, though N/A should last a lot better), even 150,000 but would be reluctant to buy a car with 200,000 Km on it. There's a reason it is only 6.5k. If you're willing to spend 15 then look at all the cars and buy the one that is in the best condition with the best history which kinda goes without saying. Skimping now could lead to expensive repair bills later on as boxer engines are a pita to work on. Also dealer car should come with a decent warranty (12 months i would assume).
Get a decent one with reasonable mileage and stick to service schedules and you should be fine with the car. I have an Impreza ('99 STi) and it hasn't put a foot wrong yet and is also my daily driver. Oh, and it's a lof fun in the snow.
Whatever you get best of luck with it.