What are you looking for exactly? How much coffee character? How sweet? Any particular flavors or attributes you want? Your recipe as is will make a fine beer, but I can’t know if it will be exactly what you are looking for.
If you want something creamy/sweet I would go with a more neutral bittering hop than Chinook (pretty much anything else). What are you trying to get from the Challenger addition?
I also like darker crystal malts in stouts, maybe a caramunich, or crystal 80, something like that (even a blend of a medium and something really dark like 120 or special B if you want some dark fruit character).
I might hold off on the lactose until the beer ferments out. You can always add more/less depending on how it tastes.