My thoughts:
18 vs 17 won't physically alter the wheel/fender gap as the overall diameter should remain the same. BUT ... the larger wheel could *visually* reduce the appearance of the gap by bringing the metal bits (wheels and fenders) closer to each other, with less rubber in between. Note that this visual effect only works if the wheels are visible, i.e. not black. Your plastidip idea (assuming you mean black plastidip) would competely negate this. Black wheels just make your entire wheel wells into fender gap (maybe that's the theory - if you can't see the wheels, you can't have wheel gap
).
Otherwise, both wheel sizes will work. Just remember xdrive needs the overall rolling diameter to be the same to save the transfer case. If possible, consider a square setup, this is best (though not essential) for the xdrive, allows tire rotation, and can help reduce the inherent understeer.
Aftermarket vs OEM doesn't matter. At all. Ditching the runflats is good as long as you're ok with what to do in the unlikely situation that you get a flat.
Avoid black wheels.
*Edit: no, autocorrect, I didn't type "gender gap" twice...