ok mate, for out-call and it being first time I'd strongly recommend an established agency such as Diamonds, Amour, NNE or similar.
This is mainly for your security/safety as agencies always monitor the girls and there's a lot less chance of problems with agency girls on an out-call.
It's nearly always a minimum 1 hour booking for an out-call and the agency will ask for your address/ phone number etc. for the girl's security etc.
The profiles on the agency web-sites will tell you all you need to know about age, sizes etc. and most of them will have reviews too.
If it's late at night some of the girls will have a driver, but he/she will stay in the car and should not cause you any problem.
The girl will phone her agency on arrival and possibly before leaving just to let them know that everything is ok, again that's nothing to get concerned about and quite normal
I've had several out-calls and find them to be much more relaxed and comfortable than in-calls. Just make sure that the heating is on and offer her a soft drink, not alcohol !