Getting 18 inch wheels wouldn't drop it. You'd just get tires with more sidewall to compensate.You're going to park it outside in the snow? Get a cover. Ground clearance, don't have my tape measure, but I can put my fist under the front just barely and mine is a stock 2014 (hammer fist - thumb up) if that helps. Stock meaning no suspension adjustments at this time. But if you're worried about ride height, why would you put 18" wheels on it when it comes with 19" ?? Dropping it a half inch...
I've lived in Boston, Park City, UT so I know snow, but now i Live in San Diego and I don't even go out in the R8 if it's raining.
Anyway, I believe we have somewhere around 4-4.5 inches of clearance stock. I would definitely not park outside in the snow, though. I'd imagine you wouldn't have much trouble driving around if roads are plowed at all.