I guess it depends on how strong a climber you are then and how much time you have to do the flat bits. Gear too high and you'll be walking, too low and you'll be taking a long time. I've ridden 65-70" gearing around flat London and found it fine so I'd be looking to go lower than that with the cols involved and the touring aspect - you're not in a rush so better to do it comfortably on a lower gear than grind your whole way through the trip. Maybe find some examples of the grades you hope to ride closer to home and ride up them a few times.
I guess it depends on how strong a climber you are then and how much time you have to do the flat bits. Gear too high and you'll be walking, too low and you'll be taking a long time. I've ridden 65-70" gearing around flat London and found it fine so I'd be looking to go lower than that with the cols involved and the touring aspect - you're not in a rush so better to do it comfortably on a lower gear than grind your whole way through the trip. Maybe find some examples of the grades you hope to ride closer to home and ride up them a few times.