I am considering buying a 1999 E300D and the current owner says that the glow plug light is on.

The car has 227K miles on it.

He also says it starts and drives fine but I'm not near the car to verify that.

If it isn't just a glow plug(s), what else could it be and how much would it be to repair?

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1993 W124 300D
-297K on the clock as I type this.