Viper, being best friends certainly will make it better, but it's still no guarantee. As others have said, you'll change a lot in the next few years. I'm glad you'll be waiting before going from best friends to engaged, and that you'll be putting the bf/gf stage in there as well, because there are differences between that and best friends.
Staying engaged for a long time after that is also a very good idea. If you can last that long, and at the end of it you both want marriage, then you'll likely have a good life. And if you can't last that long, then it probably shouldn't have happened at all.
Stick to your guns about physical intimacy. If it's what you want, then it's right. If later you both decide that you don't feel you need to wait for marriage, then it's okay to change your mind, as long as it's what you feel is right for you both.
You both will definitely change in the next few years, so don't be too surprised if it doesn't last and the marriage doesn't happen. You especially are likely to change. You're just getting out of high school, presumably going to college. You'll be meeting new people and having new experiences. However if it does last, then good for you both and I hope you're happy for a very long time. (She's cute and you seem okay and you like her so she's probably okay too and it's always nice to hear about a couple that's cute and nice and lasts a long time together.

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When I first moved to Massachusetts, I thought for sure I'd be marrying my girlfriend soon...we broke up within a year and a half because we found out just how different we were. During that time I reached a point that I thought we might end up married anyway just because I didn't know how to get out of it, but we finally ended it because we nearly wanted to kill each other. Don't let something like that happen to you. You've been 'together' for a while now, but not in an actual admitted relationship, which will change things. And as the years pass and you both change, you may come to find you're no longer on the same path in life.
My mother was married twice. One guy, my father, she divorced because he's a psycho (and unfortunately I take after him in too many ways). The second husband died. Then my aunt introduced my mom to someone. He'd been married twice before to nutjob women. They dated for six years before he finally proposed, and they've been happily married for about 4 years now. I don't know that it would have lasted if they'd gotten married sooner, or they may have been a bit more unhappy in the marriage. She was still responsible for both of my brothers up until a short time before he proposed, so that would have gotten in the way. And they both needed to decide if they wanted another marriage. But they took the time they needed to make the decision and to make sure they both knew what they wanted from each other.