I had a '77 300D (same engine) and this seems a bit too long, to me. In the 50's the glow plug light should stay on perhaps 15-20 sec...if that. On my '83 300SD recently I had to use a wire brush clean corrosion off the negative battery cable and the terminal block where the positive cable distributes power to the rest of the car. Between your glow plug lag time and your starter cranking time, this would be the first thing I'd check. Otherwise I'd *guess* you may need a glow plug relay, but I'd check the terminals first. Cleaning them can't hurt.