Jamie you seem pretty clued up on the cars m8, but keep an ear out for any piston slap coming from the engine, one of the impreza let downs, along with any other knocking or spurios sounds.
look for signs of accident damage ect.
Check out the condition of the tyres, discs and pads, and the clutch, becouse to replace all these could easy set you back 1,500 quid plus.
Test the car out on the open road, pay attention to the suspension, check them for leaks, this can be another costly thing to take care of, as alot of imprezas are used for fast road/track use. and are hard on the things ive mentioned in this post.
Pay attention to how the car comes up on boost also, and to wot it boosts too. this will give you a good indication of how the turbo is doing/perfoming.
Under the bonnet look for any signs of oil leaks, rocker covers are quite bad for it as are crank seals, and power stering pump seals/tanks. Check when t/belt was replaced.
So welcome to the club m8
and happy hunting for a scooby.