Leave it up to your girlfriend/wife to select
Men are easy to please, it's the ladies that are a little trickier.
If you want your partner to have an amazing time, and especially if you are hoping to continue to indulge in the fantasy, it is imperative that your partner is very attracted to the companion. She needs to be eagerly anticipating the liason, find the lady to be sexually attractive, and feel comfortable with the situation.
If your partner is having a wonderful time, you will have a wonderful time!
My suggestion would be to have your wife or girlfriend peruse the adverts and select a few ladies whom she finds to be appealing. She can then call them to see if she is comfortable with their demenour, and to find out if they have a good repoire.
Have fun!