…I’d look for someone in their 50s or so, with blue eyes, who could invoke the standard anime crazy grandpa. Either he’d already have red hair, or it’d be dyed for the flashbacks. Then, for the regular scenes, he’d wear makeup and have his hair dyed white.
The Dumbledores in the existing Harry Potter movies don’t really work for me. Hai has pointed out that the first one seemed way too frail, and the second one seemed young but weird. I think they were both too solemn. Dumbledore is nuts! That needs to come out from the very beginning.