You can approximate a friendly relationship with some rare providers, but YOU ARE FOOLING YOURSELF if you think you are moving into anything like a girlfriend type stable situation. If they sleep over it could be simply that they are tired and they think nothing of sleeping over at a client's place.
As others caution, be very wary of her but also of your own powers to rationalize. You can be a friend, but you are NOT ... not not not special to her. Even if she says you are special to her, she is probably lying to you so that you take her on a trip or something.
Most of these girls are girls are either fucked by daddy at a young age, addicted to heroin or cocaine, or in some other way only used to sex as a way to lube the wheels of life. Be nice to the girl, pay your bill, offer her token favors, sure, but this is not going to segue into a situation where she'd stick with you if you are unemployed for several months. You will always be a means to an end.
In "normal" relationships men use commitment to get sex and women use sex to get commitment. In this equation it's men using money to get sex, and women using sex to get money. As long as your money is spent on gifts that she can keep (versus gifts such as tickets for both of you to see a show), you will be viewed as a source of income. Even if she *thinks* she cares about you, she is probably just rationalizing away the fact that she's renting pussy.