you could avoid one way air fairs by seeing if you could buy an ex hire one once your over there?
For my personal slightly pessimistic outlook I can see a multitude of problems, there are just too many unknowns. If you buy out there and discover its knackered and hard to fix you can ram it down the throat of the shop you bought it off. Or you might arrive the week after the head the great morzine 2nd hand bike sale and all the bikes are gone. Also How are you going to sell it back again, or do you bring it home? The tactic to getting a good price when you buy or sell is to bide your time, you wont have much of that on tap on holiday.
As a do it myself kinda guy I would buy a couple of months before and make sure all was well and get it set up to suit me to prevent any last minute problems ruining my 365day longed for trip whilst fixing up an unfamiliar bike/waiting for parts/dealing with gallic nonchalance/trying to find a correct length stem to suit your odd shaped body..