Welcome to the site
Nice cars but as its a new(er) car unless you store it and don't drive it, it will only go down in price.
Any car will benefit from a free flowing exhaust depending on what you want the car to do. An exhaust on its own will only give minimal power increases but will sound better.
£31k seems par for the course but it's supply and demand, always try and get something knocked off, especially as the market for an "as new" WRX STi is quite limited, a bit specialised for most people