Will I be charged some exhorbitant fee on arrival, or be told that the car I wanted was no longer available? I've not seen thus far where I can reserve without a prepayment.
I wouldn't expect any exhorbitant fees, but it's possible there might be fees such as the road tax or train station pickup fee, and maybe even a modest one-way fee (when I've paid a one-way fee in France, it's been pretty modest, and I knew the amount in advance). Check your voucher or confirmation to see what it says. I generally rent through AutoEurope, and the voucher clearly states what will be extra and approximately how much.
As to no prepayment, while AutoEurope always requires a prepayment (refundable in full up to 48 hours before the rental period), I do remember one time renting directly through Europcar and having 2 choices at the time of reservation: one price if paid in full in advance, and a higher price if reserved with payment to be made at the time of pickup.