UDK & Cryengine 3 are decent to work with and both offer existing, free to use code that people can expand on. But if you want the task of basically building the code from the ground up, then go Unity. EDIT: JKA is so cheap now, that anyone who wants it should be able to get it. I think just have this hydroball be only for those who own JKA. I've been apart of 3 Star Wars projects on 3 different engines, and 2 have died horrible deaths and I'm not sure what's going on with the 3rd. So I wouldn't recommend starting one yourself.