Well, it seems that you will take a taxi from downtown Zhuhai to Guangzhou Airport. If so, I think it is much better to book a car from the car rental company in advance, because less cabs in Zhuhai could be hire to the airport on the spot. By this way, the driver will be trusted.

If you leave before 18:00, I think you could catch a coach from Zhuhai Gongbei Bus Station to arrive at Guangzhou Airport directly. The journey is about 2.5 hours.

As for a hotel at Guangzhou Airport, you could consider the 2-star rated Renhe Good East Hotel (3kms to the airport)and the 3-star rated Airport Hotel (1km to the airport). Both of them supply free shuttle bus.