contact a local chartered car company in Guangzhou or book a local free tour. The travel agency will help you find a car. I think the latter one is easier for you.

You can check out www.guangzhouassistant.com. She got me a Toyota Camry with a driver for 8-10 hours recently for a very fair price, around 5-600 rmb if I remember correctly. I'm sure she can find a bigger car (or cars) for you. You could also rent a taxi for the whole day - it is not very expensive.