I’ve never heard that about Colin Farrell, but it is pretty widely-known that Stuart Townsend was originally cast as Aragorn. He started shooting, but they decided early on they wanted someone older, with a more commanding presence. The rest of what you wrote about Viggo is true.
Stuart Townsend is probably best-known as Dorian Grey from League of Extraordinary Gentlemen, and I believe he was also in Queen of the Damned.