Glow plugs seem to last 10 years or more on the older diesels. My 1987 still has its original plugs @ 125K miles, though they are getting a bit tired.

Things are different with the OM606 engine - think '98 & '99 E300 turbodiesel. The original glow plugs on these cars rarely seem to last more than three years. I had mine replaced under warranty at about 35K miles, I think it was.

Run away from the $1K quote, it's absurd. Changing the plugs on this car is more work than the earlier diesels, but it still shouldn't run more than three hours shop labor, at most. Even at dealer pricing I can't imagine paying more than perhaps $600. It's a pretty easy doityerself job as well, with parts being $120 mail order.