Had an R53 Cooper S since new, brilliant car.
Brother had an R56 - very nice, a bit more grown up but a bit less fun than the R53.
Ride is always better on the 16" wheels, lose the run flats and the car rides perfectly nicely - R56 better than the R53.
Steering should be epic. My R53 steers better (more direct and more feedback) than my 996 (which has been fully refreshed at the front, alignment done recently too). So if it doesn't steer right, walk away from it - should be great if the car is a good 'un.