You start where you want. The story of Doctor Who is always somewhere in the middle, even in the beginning. If you want to start at Rose or An Unearthly Child, sure.
I think the Five Doctors would be a great start to get an injection of Classic Who, then the 50th Anniversary multi-doctor story. From there you can go anywhere because a short couple of episode introduces the base personality of a lot of Doctors in a short time.
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