If you choose a wheel that was available as original equipment on your vehicle then there should be no issue with the brakes, and although fitting smaller wheels will have a fractional effect on the speedometer, this would be partially offset by the increased height of the tyre, and would cause your speedometer to indicate a higher speed rather than a lower speed, which is the better outcome in terms of legality. You may need to try a different dealer or source the wheels from a parts specialist to avoid this issue.