It sounds like you are trying to make the car a bit more special to drive so from my own experiences I would replace the exhaust back box for a Sportex item just to make it sound and look better, replace the wheels with the ones from an ST170 with some good tyres and replace the seats with some RS items. This will change the whole feel of the car and although RS seats are expensive, keep the originals so you can resale them when the time comes. Only other mod worth doing is to replace the front hubs and brakes for St170 items and then possibly fitting lowering springs.