with the same offset, +0.5" on wheel width wouldn't change a lot the driving behaviour. the wheel would stick +0.25" farther outside. the added weight is the only thing that MAY be perceptible, assuming you're having the same tire width on both setups.
concerning 17" vs 18", there is no difference assuming you're having the same overall tire diameter
On the iDrive system, there is no problem, if the 4 tires have the same overall diameter, and that the TPMS is well placed. on the xDrive system, no problem, with the same conditions. the speedometer and odometer will be off, though.