This isn't really about your daughter; it's about your own misgivings. You are the one finding reasons for her not to go; if you ask her, what's she going to say?
If you ask like this, she'll surely say no and you can be off the hook and say SHE didn't want to go:
"Honey, do you REALLY want to do this party? I'm asking because you've never slept away from home before. You know Birthday Girl but there will be a lot of kids there you don't know. If you want to come home it's a WHOLE fifteen minutes away. Are you really, really sure?...."
That's how to put her off doing it, by planting all your adult worries in her head and making her doubt what should be a simple, fun choice.
If you ask by saying, "Hey, what do you think about Birthday Girl's sleepover idea?" and stop right there, she is likely to say yes.
You are anticipating the worst as hard as you can: She might need a pickup, there will be other kids she doesn't know (isn't that...life?), she is not super-best-friends with the host kid, YOU are not super-best-friends with the mom.....Yes, it could all come true! Your daughter could get homesick, a giant shark could scare her, the birthday girl might give attention to other kids for a while....but why not assume the best and if you do end up bringing her home, heck, a lot of parents have done that one. She is more likely to make friends among the new kids; be so occupied and busy with the activities and the amazing setting that she never once misses you; and be so busy that she crashes out and sleeps at some point.
If you are worried about the "never spent the night away" thing, tell the host mom that you are. Ask her if maybe your kid can sleep near the girl or other kids she already knows, or in the mom's vicinity so mom can hear her in the night. Beyond that -- let the worries go. Even if some of your worries turn out to happen in the end, at least you won't have planted them in your child's mind in advance and scared her off the party.