Hi,
you can get by with a hybrid bike like the ones we use (mine 300 euros used my wifes 300 new details, pics and tips on our page). For a second hand bike make sure it fits, has low enough gears, the wheels are double walled with 36 spokes (let your local bike shop check them out), wide enough tyres and a comfortable saddle.
I would not go for a road bike.
Southern France will be busy in summer, read some journals from www.crazyguyonabike.com. To avoid traffic try going inland a bit but that will mean hills.
Finally go to google maps and plan your route averaging 100kms per day with a rest day every 6/7 days.
cheers