make sure the chat system is behind https:// so anyone sniffing your chat port number is not getting inappropriate information. Make sure that both the client and your admin tools receive the chat log directly for later reference in the log or by ticket number.
don't worry about the client grading the chat support, often they're just commenting on the service not the answer.
any request too sensitive to your business to do on chat, even securely, should be done over the phone, ie: where is your largest area of fraud? Maybe it's John pretending to be Jane?
Don't bother with pre-scripted phrases, let the csr's input their own over time. They do know what to do and what they like. This can then update organically.
chat takes forever! so little phrases like ...is there anything else I can help you with? etc moves it along a bit faster.
have the csr grade the topic of the chat after the chat is closed so you have those statistics. If you're having problems in one area this will show up easy enough. Then update the website information accordingly either once a week or month, but no later. No point in saying the same thing 500 times a week in chat if it can't be said properly once on the main website.
This also means that your clients control your content as it applies to your sales, and you're just supplying a design interface known as a webpage ;) Consider also wiki-type interfacing to the website for the csr department and quick updates.
good luck | 
