I wouldn't be interested in buying a used WRX at all.. or any other turbocharged four cylinder for that matter. Too much risk involved IMO (been there, done that).
If you are considering buying one I would shoot for these criteria: 100% stock its entire life, one [mature] owner, mechanically in good condition, well maintained on schedule, under 150k miles, passed a used car inspection (including leak down test), and you can get it for less than $6500. Basically you're going to be looking for a gem.. however they do exist. It just might take a while to find the right one. Once you get in the 12+ years old with 190,000+ miles range I wouldn't spend more than a couple thousand at most.. you would want to have a back up car to drive, and if something on it were to break that would cost more than $500-$600 to replace just write it off as a loss, part out, and scrap what's left.