I'd lay bets on the starter solenoid. Starter isn't that far behind, either, it it's original.

You can test by checking for battery voltage at the small connector on the starter solenoid when it won't crank -- if you have voltage, the solenoid is bad, if you don't, it's the key switch.

If it clicks, but nothing happens, the contacts at the solenoid for the starter motor windings are bad, replace solenoid. Might also be worn out brushes, in which case I would replace with a rebuilt starter.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!