I am a kind of nervous type in normal situations never mind being with a WG.
I am innately a nervious person too and I am like that to this day on every punt. In a nutshell don't worry about it. its one of the types of people who book these girls.
I would need a women who is going to be reasuring and make me feel at ease if that is possible.
You are explaining what the bare essentials are of her job. So in effect any girl that is relatively ok at this job will be fine.
The only advise I would give is two fold.
1. Don't linger on things, set a day for a punt and do it. That means don't book in advance find someone availalbe that day and book - so that means either someone down as working today on adultwork, or booking via an agency or parlour.
2. Don't whitter on for the booking. As in probably don't email. Just phone, say hi I am bill I want to book you for 2pm today are you available. And then thats it. Cover the 'I am a new guy to this and very nervous' when you have the money in your hand but not giving it to her until you have had her say thats no problem. Then give her the money. If its a problem for her she wont take the money.