You can certainly buy a French SIM card in France, I am also a frequent visitor to France and carry an SFR SIM card, which is pay as you go. Just be aware that they have a limited life, a "use it or lose it" unlike my UK SIM card which retains the credit.
You will probably be asked to provide ID and an address to register the card.
Orange are notorious for locking their handsets, but I believe that an internet search will assist you in unlocking the 'phone and making it available for you to use with other networks.
You might also be interested to know that you can buy new backs for most 'phones that will allow you to carry both SIM cards in the one handset, and you select which one you want to use when. (You can't use both at the same time in the one handset). This cuts down on the number of mobiles that you need to carry around. Again a quick internet search for "dual SIM covers" will point you in the right direction.
Hope this helps.