You're already over thinking it all. Just call whichever girl you like the look of and have researched a little (reviews, asked around in chat etc) and ask her if she's free in or around the time to require. Ask her how much (although I never do as it clearly states on most ladies profiles what they do and don't do and how much they charge) she charges and so on. If you want her to wear something in particular, probably best to ask early to avoid disappointment.
Next thing you'll wanna do is call closer to the time to confirm and get exact directions. Be ready to have a shower. If your long hair is an issue cut it or buy a shower cap. I doubt they'll laugh in your face if you wear one, they've long hair too so I think they know what a ball ache long wet hair is. Remember these ladies have to wash themselves a shit load more than you so I'd be willing to bet one or two of them have shower caps too.
When I call first I always say the same shit. "Hi, XXXX. How are you? I was wondering if you were free at all between (whatever time suits).
It's all fairly straight forward from there. Arrive on time and be polite. You'll have a ball.