$50 is a good deal for the bike if it's in good working order. Cottered cranks are fine, just heavy. Inside ring is probably a 42T so use that. If you want to go singlespeed you can keep the back wheel, just have it redished. Once you start replacing wheels and cranks, you're better off with a Kilo.

The extension levers will slow you down but don't rely on them to stop you quickly. I'd get some new Kool-Stop brake pads. Braking in the wet on steel rims is not good, give yourself plenty of time. If you plan on skidding, this doesn't matter. Just make sure the wheels are true and the spokes have even tension and they'll work fine for your project.

/I had bought a '76 red and black Grand Prix for $40. Put new tubes/tires, brake pads and bartape on it. Sold it to a friend for what I had in it. Bike rode decently, wasn't dead like an old Gitane I'd found.