The starting issue may be your NSS switch. However first I would look at the grommets(bushings) in the transmission where the shifting rods go. When they wear out, the tranny does not know exactly what gear you are in and this will prevent it from starting in Park.

I have a 1981 300SD and had the same problem that you described. I thought about replacing the NSS switch but inspected the grommets first and found that one had completely disappeared (the rear one). I replaced just that one and magically the starting problem disappeared.

Good luck. Easy problem to fix if this is actually the cause.