Personally id go for a tdci sport. I looked at st170's nearly two year ago and found they were expensive on insurance, fuel, tax and not even 170bhp as quoted. I went out and bought a imperial blue tdci sport.
They have the looks of a st170 and with a remap leaves st170's in a cloud of smoke. They are cheap on insurance, tax and get good mpg. Also the torque and turbo factor makes them great fun to drive.
I love mine to bits and as its still a joy to drive.
they can be had for a good price nowadays although imperial blue models are rare as only 500 were made.