I have a chance to get a deal on a winter rim/tire package but unsure due to the size differenece of the tires they come with and the fact that I know nothing about "offsets" and "spacers"

The stock all seasons currently on the G35X S are 225/50/18 and the package deal are 225/50/17 210 Pirelli Snowsports on 17' 6-star Eagle Alloy Rims. The previous owner had these winters on an '03 G35 Coupe.

Will these fit my car without anything else needed? Is the size difference b/t 17" and 18" a concern?

Any advice is appreciated

I think you'll have some issues. The stock 17" rims need 225/55/17 tires. So the aspect ratio is wrong already. So, the diameter of the tire is off already. If you change tire to a 50 aspect ratio, then you might have a chance. You'd still need to check the offsite and bolt pattern as well.

However, having to buy new snows with a 50 aspect ratio probably defeats the purpose since you could buy aftermarket rims new, for $400-600.

Good luck.