Ater the game will install you can use remaining 9 GBs for something else. Simple solution - get 500 or at least 320 GB 2.5" (laptop size) hard drive and upgrade drive in your PS3 and you should be fine for a while. If you'll purchase some nice external USB enclosure at the same tiem you can use the original PS3 hard drive as backup for your PS3 or laptop or any other needs
The game itself does NOT require 18 GB - you just need to have free space on the hard drive twice the size of the game to be able to downlaod and install it. It's the same with any other game or demo. It demo lets say is 1.5 GB you'd need to have at least 3 GB of free space to be able to download it.