Kids are usually tired from sleepovers-- they often are so excited that they stay up late talking and playing, and are exhausted the next day.
What might help you find out if your daughter is ready is to ask her in a day or so 'So, how was it?" and see what she comes up with. Are her comments more positively or negatively based? Is she still dragging when it comes time for school tomorrow morning? Those are some of the things I'd think about when considering sleepovers in the immediate future.
In my opinion, not quite 8 is still a bit young, and kids could use a couple more years life experience, because visiting a different family without one's parents is sometimes like visiting a different country. Different foods, rules, etc. Some kids roll with this, and some kids are thrown off because they tend to thrive on familiarity.
For me, I didn't really do friend-overnights until I was a bit older. My son is three and while he's asking for "sleepovers" (I don't even know how he thought of this) we're likely going to wait to do any non-family slumber parties until he's 9 or 10. And we'll likely try it out in summertime, so he can sleep in later.
(PS- from my work as a nanny, it's pretty common for the parent of the visiting child to do the travel both ways. At least from what I've observed, there are no hard and fast rules.)