I have the Base/Touring model and mounted some Blizzaks from the local Big-O Tire store on the stock 18" rims (got mine powdercoated). They're ready to go when the snow comes.
The 18" Base model rims are a bit narrower than the 19" Sport rims so if you can get your hands on a set of these rims some narrower winter tires will fit great.
I went with the stock Base tire sizes (225 in front, 245 in rear). My Z is my daily driver but the wife drives a 4x4 SUV so on bad snow days I'll be carpooling with her.
You could also go with 225's all around if you want a little better snow traction, the 1" wider rear base rims will still handle 225's just fine.
I wanted to keep some stagger (wider rear tires) to maintain handling and performance on the non-snow days during the winter.
The tires were about $800 (tirerack.com had them on clearance, Big-O matched their price) and base rims seem to go for $1000 and up, so the $2000 figure quoted above is right on.