My wife has a 15’ X5 too and I’m pretty sure the smallest wheels that will fit over the front calipers are 18”. I did some searching on the bimmer forums a couple years ago because I initially planned to buy a dedicated snow tire/wheel package and the consensus was 17” wheels didn’t clear the calipers.
Most BMWs come with runflat tires and no spare. So keep that in mind. If you don’t have the optional donut spare, you’ll have to buy runflats. If you do have the spare, you’ll need to keep the tire size consistent, or you’ll have to buy larger spare.
We bought my wife’s with the “xline” package, which is the kinda-sorta “off-road” package. It came with 255/50r19 Continental 4x4 Contact runflat tires. They’re hardly all terrain tires, but they’re a little more aggressive than sport tires typically on X5s. They perform great in the rain and snow, however they’re almost $300 a piece to replace.
Side note, if you’re still under warranty, I would not mess with the tire size. I can’t imagine BMW dealers would be be receptive to that.