I used the Mines R34 Skyline GT-R and the Tuned Elise 111R to complete the S-Class.
However I have heard that the tuned Ford GT is VERY quick, but it can also be very difficult to handle with all aids off....
I would say turn off autosave, buy the Ford GT, test it, if you like it turn autosave back on. If you dont like it then just quit the game and start it again and you'll be back to square 1 (note that auto save will be turned on again after restarting unless you manually saved the game while it was turned off!)