I think Colin Firth will be fantastic as Henry Higgins. Even though Colin Firth is over 50, I think he can play younger. I picture Higgins as, at most, in his early forties, and he doesn't really look older than that. I'm very glad to see a return to a sexy Higgins.
As for Eliza Doolittle, what about Samantha Hill or Mary Michael Patterson? They both have the singing voice and the acting chops, and they're the right age. I've seen both of them as Christine Daae in POTO, and they both blew me away.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.