I was a big UNCLE fan when it first aired, I was even using a reel-to-reel tape recorder during the first season. I've often considered getting the series dvd's, but I know the 3rd season is god-awful and they couldn't save it in time during the 4th. I had a bunch of the novels, the Man From UNCLE magazines that had short stories, the AMT UNCLE car, and the Aurora model kits of Solo and Illya. I saw at least two of the feature-length films they assembled in theaters. It sort of primed me for Trek fandom.
I don't know enough about what actors are hot-or-not these days, but age casting shouldn't be a big problem. Vaughn was older than McCallum by only 10 months.
And Illya during the cold war? In one interview I read, it was either with Norman Felton or Sam Rolfe (probably Rolfe), it was suggested that if the Soviets had ordered Illya to kill Solo and come home, he would have done it without a second thought.