1) No worries using the original BB parts as long as they're smooth.

2) I'd lean toward keeping the Stronglight crank; it's likely as good or better quality than any inexpensive modern replacements you might find. What model is it? Orphan arms often appear on eBay.

3) Chances are your steer tube is also French thread, which would make replacing the headset problematic unless you also replace the fork. If there's nothing wrong with the original headset I'd keep using it. The crown race is the most likely part to show wear. If it is worn, you may be able to replace it (or both the crown race and lower pressed cup) with a similar race from an ISO headset and keep using the French threaded parts.

4) Are you planning on buying specific SS/FG wheels? If so, simply running the chainring on the inner plateau of the crank often gives a usable chainline with no further adjustment.