40 year old car
Repainted at least once
Some rust
Some old filler
You want to treat the car with respect

IMHO, you have no choice to take it to bare metal. Its not that hard, just get friendly with a DA sander, lots of sanding disks, and prepare to get dirty.

Once its stripped you'll be in a MUCH better position to assess the condition and repair the metal that needs repair.
If you DON'T strip it down, how are you going to know what's under that paint, and the paint you lay on top?