Actually, in my opinion, the Simplex derailleurs are just fine, and work well. However, they are prone to catastrophic failure, the front one breaking in half and, often times, feeding itself into the crank rings. Messy.
The recommendation to go to a Suntour transmission is a good one.
Since you are new to the interest of vintage bicycle ownership, you might also be interested in spending a bit of time at a website aimed at helping people find and street restore old bikes. Please feel free to visit,
MY "TEN SPEEDS" and, perhaps, save yourself some money and effort as you build up that
nice old Peugeot.
Hope this is a help.