This is a great project and no you don't need to sink much money into it to make it work. We can talk you through this. You have most of the bits you need on your bike so you just need to rehab them. The bottom line is that all the parts you need for the bike are relatively easy to find and can be found relatively inexpensively.
By the way, I'm a big fan of French bikes (as are a lot of posters on this site) and I've rehabbed my share of them. Plus there is a lot to be said for riding your father's old bike. I do. I maintain and ride a 1993 Bridgestone XO 2 which was my Dad's bike.
We'll need pictures of your bike and an idea of what kind of riding you would like to do on it. Great story on Sheldon Brown by the way. His website is full of invaluable info on old bikes.
Last edited by bikemig; 08-01-20 at 05:08 PM.