I'd would definitely say that even just kissing while drunk is not cool for a relationship. If you can get her to stop that, then o.k. For this effort you had better like her for more than everything/anything sexual.
15 and admits to a bit of fooling around. That's a tough pill to swallow. You will HAVE TO trust her, considering this. Otherwise, your paranoia of her fucking around will just make you unhappy.
She better be really damn attractive, or be disturbingly kinky for all the effort.
Also consider that she has sex outside of a relationship. This kinda says that she doesn't need alot of effort to get to sex. Kind of a warning there as well. Plus, Randomly making out has its consequences also. Cold Sores, a form of Herpes, never go away. The actual scars do, but not the infection.
I guess no one's perfect, there are alot of flags here, and no one here knows her like you do. She could be the sweetest girl for all I know. IF she's a fun person besides sex, and you could trust her and do believe she'd not gonna mess around, well, there you go.
Seeing as she's 15 though, You should keep in mind that girls that age may not be ready for a longterm/serious relationship, even if they think they are. When I was younger, I never knew an totally emotionally stable girl my age. Be ready for the bottom to drop out at any time. Not saying that it will happen, but considering everything, it would make the break-up not so shocking and depressing.
Id be asking myself, considering everything, "Is this/she all worth the good and possible bad?" cliche but true: "Not the only fish in the Sea"