So many good questions

I don't have a speech prepared, although I know many VAs who do use them - I have a basic outline in my head of the important points I know I need to get across but otherwise I ask the client some questions first to help me better understand where they are in their perception of what I can do for them, etc.
From there I illustrate how we can work together on a few of the tasks they've mentioned, letting them know that I'm experienced in helping clients delegate what needs to be done, etc.
That confidence that you can help acclimate them to a virtual working relationship goes a long way with uncertain clients.
As far as turnaround time we don't guarantee anything but generally try to work on 24 hours turnaround with a 48 hour window (for basic admin tasks) not including weekends and projects that come in after a certain hour on friday, etc. etc.

. There are some phrases regarding this in my letter of agreement but nothing with a legal commitment or guarantee.
One thing to remember is that your client, if he's very new to the concept, is likely even more nervous than you are - your own strength of character and steady voice plus well articulated information will help him get excited about working with you.
You can also ask the potential client if he has any questions as you go along - and use his questions to highlight your own strong points and service offerings.
You'll do great, I have no doubt!