Jays I'm not the most experienced punter on here but there are some good tips that apply to most situations.
Firstly see if she has any feedback, on the site she advertised on and, probably more objectively if anyone on here has reviewed her.
You are right to find a girl who matches your requirements, but as above, find some feedback/reviews that confirms she deliveres as advertised.
Don't know what your financial situation is but £250 for a first punt seems a lot, bit of a leap of faith. There are loads of girls on AW who offer the same and you can book for £60 - £80 for half an hour, depending on the girl and where you are. Same rules apply though, check them out first.
You have opted for an agency girl. Again, loads of independents available who don't have the same overheads and are therefore better value.
I'm sure someone will give you a review on your girl, but just be careful until you know exactly what you're dealing with before you splash the cash, especially quarter of a grand!