Go with whatever you already have would be my choice. Depends on the tour, credit card, self contained pannier loaded, off road, or day tripping with groups.How many miles a day, overall mileage. Sounds like a great time to be had. My neighbor tells me I should do a bike tour in France. ETA: I just saw your above link, this looks like an organized tour with support and full itinerary Any good road bike should do the trick. What are your choices so far. This rental looks good http://breakingaway.com/rental-bikes/
I do agree that if your planned tour is fully supported a good fitting road bike will work nicely. If you now use clip-in pedals I would bring them and the shoes you use. Bringing your own saddle is also a good consideration. Another thought is a saddle bag that is large enough for your on the road daily needs. I have toured France several times and it is a wonderful place to see from your saddle. Hope you enjoy it as much as I have.
I would take my own seat/pedals and a cloth tape measure. I would then measure and mark the distances from the pedal down at 6 o'clock to the top of the seat and then a measurement from the stem or back of the handlebars to the front of the seat. It really doesn't matter how you set you the measurement up as long as you know how you are measuring and then get the same measurement over there.
