Hi and welcome. I'm in Dorset and I took my blob to Alan Jeffries (thinks that was his name) in Plymouth. I highly rate them. They will supply and fit anything to do with a scoobys.
Where abouts in the southwest are you?
I'm no expert, but the standard turbo on a WRX (assuming it's not an sti) would be limited to 280-290bhp. To achieve the 320bhp that you say it's advertised you would need more than just a re map. Turbo, injectors, air filter etc. So I would check what mods have/not been done. If the car has had a remap the owner should have a dyno printout to show the figures, but again with a tdo4 turbo you will not see those figures. And yes any power gain that you want, most will need a remap.
When looking at car check the underside (wings, sills, subframe etc) for rust. Check oil, water, tyre wear etc. The usual. Would be good to have a fellow southwesterner around and maybe have a run out. Cheers
Oh and experts, please correct me if I'm wrong. Thanks