the only thing more risky about rwd in snow vs FWD is turning whilst under power and sudden acelleration. Stopping a RWD is the same as stopping FWD or AWD. Stopping a RWD car with snow tires in snow is going to be easier than to stop a FWD car with all seasons.
i plan to drive my Z this winter with snow tires (dunlop wintersport m3's) on the stock 18s. However, if iwas you i'd try and wait until after winter to get the Z. just b/c its not a very fun time to enjoy a new sports car. but you gotta do what you want to do. if i was you i'd probably end up just getting it asap
but yea, with snows on the stock 18's you'll be fine.