Classic turbo wagon - Where to start?

[rosscobmw]

A good friend of mine has just bought himself a Classic shape (facelift) impreza wagon. He got it for cheap as it had no MOT, but since he owns his own garage he did all the work and now has an Impreza turbo estate with a years MOT and he wants to improve it. But neither of us, nor our friends have ever had any interest in Subarus so dont know where to start.

He'd like a bit more power and better handling, without spending thousands.

Any advice on the best things to go for? WRX or STi parts? Aftermarket stuff?