@ bastian.
The key is do not get nervous.
To prove your money you can just carry a copy of your bank statement.
I am not sure with your WHV if you need a return ticket, depend for how long is your permit valid. (for tourist people it is 100% sure they need the return ticket.)
One thing I learned about the interviews is.
Do not answer anything extra if they do not ask you. (because sometimes if you say something else that "else" allow them to start a new question, and never ends. you know what I mean.)
Good luck!.