I inset key, wait for light to go on, turn key, resistance, but no turning of starter. I go outside, bang on the starter with my hand, and it starts...what's this mean?

Yes, you need a new starter! Mine did the same thing. Read my post from earlier today.
Starter seems to hate rainy weather

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1982 300D Turbo "Helga"
380,000 miles

My '79 240 has similar symptoms on dead cold morning starts..... sometimes the key kicks the starter solenoid - sometimes it doesnt.

Am attributing this problem to freezing cold weather where the starter solenoid sticks.

With warm weather just around the corner, I'd wait and see if the problem goes away..... otherwise if this becomes a warm weather problem - I'd replace it with high powered 300D starter motor.

The 300D starter on fastlane is....134 dollars and is heavy duty...the 240D is light duty and the same price...when it comes down to it, will the 300D turbo starter bolt into place? It can't hurt to have a higher RPM starter