First a question... Do you have a Voltmeter? Can you get a Voltmeter? Good.
When the car will not start, check for power to the solenoid (term # 50) while someone holds the key in the start position. You can even check it on the terminal strip under the hood (passenger side, black plastic piece with 3 wires in it) so you don't have to lay under the car.
If you have power (10.5 Volts or more) to the solenoid, and it won't start, you know it's the starter. If you don't have power, you know that the problem is either the neutral safety switch or related wiring in the circuit.
ASE Master Tech
Mercedes Tech 21 Years
68 SAAB 96 Ice Racer
69 300SEL 6.3
70 300SEL 3.5