Okay, I don’t know where to start. Perhaps by saying that the original show compared to the movie…shouldn’t be compared. The movie is an absolute piece of racist shit and many people pretend it doesn’t exist. If you enjoyed the movie though, that probably indicates that you would enjoy the show, check my FAQ for download links. A warning though, there aren’t any white people in A:TLA, despite what the movie wanted you to think.
And as for whether Legend of Korra has some of the original cast members, yes, it has some. But not in quite the way one would expect. You certainly want to watch A:TLA before LOK.