I'd say: slap it on ... see how it looks (imagine the colours)
sleep on it for a while.
You might even come up with a two-tone scheme for it that'll look awesome, but like I say you need to see it ON the car.
As for rims on an X3, go no bigger than 16s with no larger than 205/40R16s
I have 16s on mine with 205/45R16s and they're just a bit too wide and I get fender rub. So I'm planning on getting 195/45R16s for next year.
Here's a pic of it now:
