There is nothing near airport, but subway under the airport can take you into GZ city, 18 hours is more than enough. Taxi is 45 minutes or 100 yuan, not a big deal. What is 45 minutes?
As the old saying goes, RAIN IS JUST WATER. You got lots of it in Singapore. I always travel with an umbrella.
GZ gets seasonal thunderstorm monsoon in the late afternoon, that usually goes away in 1-2 hours, when it is coming down, it is cats and dogs and chickens and pigs raining down from the sky, big time, there is absolutely nothing you can do if you are on the street, just go back in to do what you were doing before. Occasionally the thunderstorm last to late night, our flights to GZ were diverted or delayed up to 3AM, happens often. The best place to be if you run into a thunderstorm while in airport is in Pullman Hotel in the airport, huge swimming pool, spa and good restaurants. Try to upgrade to executive level with executive lounge access, free internet, etc.. well worth the extra price. Several times we watched the thunderstorm and pouring rain at the airport from Pullman window, all flights were canceled from afternoon to midnight. Be sure to check flight status before checkout hotel.