Designing and specifying an Asterisk VOIP Phone system.

I'd like help in designing an asterisk VOIPnetwork. I have 5 Avaya 9608 r50 handsets and would like to know what other equipment I'd need to set up a small network for my office. We would like an automatic answering service/switchboard (we can provide the voice prompts). Please when you make your offer ask any additional questions at the start to avoid confusion later. If you can suggest any additional features as well to optimise the use of the system that would be great.