A lot of the price variations are due to modifications, service history, mileage. As a guide, I paid £1300 for my classic turbo 2000. That was due to 158k miles when I bought it. It came with a massive folder of history and no modifications.
A budget of 8k will net you a great selection of different models. The hawk eye(2006) model does have the 2.5 engine but has a big leap in yearly tax which needs to be considered.
A local member just bought a 2004 blob eye sti with 84k miles for £6400. It's been modified to 330hp(ish from memory). Very nice car.