18" rims fill the wheel arches nicely, anything bigger is likely to knacker up the ride quality,and the Falken 452's seem the tyre of choice. Rims are very much a personal choice, but, my Z has 108's which I think look great.
I have 19" CSL's with Falken 452's or Michelin PS Cups depending on the situation. The Alpina came with 19's as OEM so I've never experienced 18's or smaller. Ride quality seemed fine to me though.