The wheel well of Millenia is very big, I'm pretty sure even a 19" would have no problem in it. As long as the offset is the correct one for your car, you should have no problem. 18x8 is a perfect size for this car, same goes for your tire size selection 235/40/18. If I have spare money, I would do the same to my Millenia. I used to own a '93 626ES, and fit a BBS RX 18"x8 rims with 225/40/18 tires, no problem.
Check out this wheel called Axis Se7en, looks very cool and size up to 19".