Nice Centurion OP lost out on one like that a few years ago but came up with a Nishiki Cresta GT, I might suggest doing a shorter tour first as a shake down which should help you figure out what works and what you might want to change before going on an epic ride. I would agree with @
Lascauxcaveman that wet doesn't have to be the death of a leather saddle. Care for it (proofide etc), fenders, saddle cover or plastic bag for the occasional downpour and you should be fine. I don't see cows running for cover every time it rains
.