I'm guessing you're going non-turbo due to your age? Its worth checking insurance to see what the difference is. Servicing and MPG costs will be similar.
It looks kind a pricey to me for an 8 year old non-turbo with 80,000 miles on the clock.
Here's a few to compare it against....
Heres a same age, similar model with only 22k on the clock for a similar price. It is an Auto granted but just using it as an example.
http://www.autotrader.co.uk/classified/advert/201406024680112/sort/default/usedcars/model/impreza/postcode/rh20pn/maximum-age/up_to_8_years_old/price-to/5000/engine-size-cars/2l_to_2-5l/fuel-type/petrol/onesearchad/used%2Cnearlynew%2Cnew/make/subaru/page/3/price-from/3000/radius/1500?logcode=p
Then if you compare to turbos you would expect the turbo to be a lot more money...but they're not.
2006 with less miles (Turbo version) and similar price...
http://www.autotrader.co.uk/classified/advert/201406074825711/sort/default/usedcars/price-to/5000/engine-size-cars/2l_to_2-5l/radius/1500/maximum-age/up_to_8_years_old/postcode/rh20pn/onesearchad/used%2Cnearlynew%2Cnew/model/impreza/fuel-type/petrol/page/3/make/subaru/price-from/3000?logcode=p
and here is a 2006 with 80,00 miles and 300bhp!! I'm not by any means recommending this car (I'd go for one not fiddled with). Just trying to show how expensive the GX is.
http://www.autotrader.co.uk/classified/advert/201402211971194/sort/default/usedcars/model/impreza/postcode/rh20pn/maximum-age/up_to_8_years_old/price-to/5000/engine-size-cars/2l_to_2-5l/fuel-type/petrol/onesearchad/used%2Cnearlynew%2Cnew/make/subaru/page/3/price-from/3000/radius/1500?logcode=p
Hope that helps mate. If you're keen on a non-turbo then by all means go for it. They are great cars but I'd haggle hard if you go for the one you have linked.