I am very excited for you and Dudley! Wishing you the best in making a decision. I believe the right age for a second dog depends on the dog and you.
My second, Teddy, was 8 months old when I brought him home. I didn't want to go through puppyhood again, and I thought Maximo would prefer to have someone closer in age. Teddy was still puppy-ish enough for both us.
Everything worked out beautifully for us. My fears of an older dog adjusting to our habits and routines were unfounded. Teddy took to his new home as if I had raised him myself.