I had a nightmare experience trying to buy a car in France. I found a car in a small dealership, but then, when I went to the insurance office, they would not let me purchase insurance for it because I did not have a French driver's license or an international drivers license (available from AAA here in the US).
I had the international drivers license when I picked up the rental car at the CDG airport, upon my arrival in France.  But, it "disappeared" - that is, thinking back, I don't think I ever got it back from the Sixt rental person. Granted, I was very, very tired and stressed at the time I did the rental, so my memory is dim. I later thoroughly searched the rental car and all my belongings, but never found it.  I contacted the Sixt office at CDG and they claimed they didn't have it.Â
Looking back now, I'm not sure why I didn't just contact AAA and request a duplicate to be mailed to me. Instead, I signed up for a "cours de conduite" in the little town where I was renting an apartment. I actually really enjoyed this class! It was me (an American woman in her late 50s) and 15 teenagers! And, I learned alot. It cost me about 250 euros. I don't recall anything about there being an additional fee for the examination.
Anyway, on the day before the exam, the owner of the cours de conduite business owner told me that she had received a message from the licensing authority saying that I would not be allowed to take the examination because I didn't have a carte de résident (I had a visa de long séjour).
At that point, I didn't get too upset because I had pretty much given up my idea of opening a small medical practice there. The Department of Health refused to license me because my degrees and certifications came from a non-EU country. My savings were dwindling fast and I was preparing to move back to the US anyway. This was such a sad, though typical, final chapter to my efforts to deal with the French bureaucracy.
I am now preparing to make a second attempt to establish a residence in France, for my retirement. I too wish to purchase a van.  My plan is to fly over several months before my move, get the van and the insurance, and then store the van somewhere so it will be waiting for me when I actually move over there. I'll definitely keep a tight grip on my international driver's license this time!