No one else bothered by "End-to-end tracking of customers" as the primary concern? Ok then. On the subject of scaling, I think it's good to have an idea in you

No one else bothered by "End-to-end tracking of customers" as the primary concern? Ok then. On the subject of scaling, I think it's good to have an idea in your head about a path to scalability. One server, using PHP and MySQL? Ok. Just be aware you might have to load balance either or both the server and DB in the future, and that's assuming you've gotten the low hanging fruit of making them faster on their own. But as this thread's top comment illustrates, learning that stuff on the fly isn't too hard. So maybe it's better to make sure you're going with technology you sort of know has had big successes elsewhere (like Java, PHP, or MySQL) and even if you're not quite sure how you might scale it beyond the defaults you know others have solved that problem and you can learn later if/when needed.